Node:Top, Next:, Up:DDD home page

Writing DDD Themes

DDD is a graphical front-end for GDB and other command-line debuggers. This manual describes how to write themes, that is, modifiers that change the visual appearance of data.

This is the First Edition of Writing DDD Themes, 2001-02-01, for DDD Version 3.3.1.

Table of Contents

Copyright © 2001 Universität Passau
Lehrstuhl für Software-Systeme
Innstraße 33
D-94032 Passau
GERMANY

Distributed by
Free Software Foundation, Inc.
59 Temple Place - Suite 330
Boston, MA 02111-1307
USA

DDD and this manual are available via the DDD WWW page.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License"; See Documentation License, for details.

Send questions, comments, suggestions, etc. to ddd@gnu.org.
Send bug reports to bug-ddd@gnu.org.


Node:Welcome, Next:, Previous:Top, Up:Top

Welcome

Welcome to Writing DDD Themes! In this manual, we will sketch how data visualization in DDD works. (DDD, the Data Display Debugger, is a debugger front-end with data visualization. For details, see Summary.)


Node:Creating Displays, Next:, Previous:Welcome, Up:Top

Creating Displays

We begin with a short discussion of how DDD actually creates displays from data.


Node:Handling Boxes, Next:, Up:Creating Displays

Handling Boxes

All data displayed in the DDD data window is maintained by the inferior debugger. GDB, for instance, provides a display list, holding symbolic expressions to be evaluated and printed on standard output at each program stop. The GDB command display tree adds tree to the display list and makes GDB print the value of tree as, say, tree = (Tree *)0x20e98, at each program stop. This GDB output is processed by DDD and displayed in the data window.

Each element of the display list, as transmitted by the inferior debugger, is read by DDD and translated into a box. Boxes are rectangular entities with a specific content that can be displayed in the data window. We distinguish atomic boxes and composite boxes. An atomic box holds white or black space, a line, or a string. Composite boxes are horizontal or vertical alignments of other boxes. Each box has a size and an extent that determines how it fits into a larger surrounding space.

Through construction of larger and larger boxes, DDD constructs a graph node from the GDB data structure in a similar way a typesetting system like TeX builds words from letters and pages from paragraphs.

Such constructions are easily expressed by means of functions mapping boxes onto boxes. These display functions can be specified by the user and interpreted by DDD, using an applicative language called VSL for visual structure language. VSL functions can be specified by the DDD user, leaving much room for extensions and customization. A VSL display function putting a frame around its argument looks like this:

// Put a frame around TEXT
frame(text) = hrule()
  | vrule() & text & vrule()
  | hrule();

Here, hrule() and vrule() are primitive functions returning horizontal and vertical lines, respectively. The & and | operators construct horizontal and vertical alignments from their arguments.

VSL provides basic facilities like pattern matching and variable numbers of function arguments. The halign() function, for instance, builds a horizontal alignment from an arbitrary number of arguments, matched by three dots (...):

// Horizontal alignment
halign(x) = x;
halign(x, ...) = x & halign(...);

Frequently needed functions like halign() are grouped into a standard VSL library.


Node:Building Boxes from Data, Previous:Handling Boxes, Up:Creating Displays

Building Boxes from Data

To visualize data structures, each atomic type and each type constructor from the programming language is assigned a VSL display function. Atomic values like numbers, characters, enumerations, or character strings are displayed using string boxes holding their value; the VSL function to display them leaves them unchanged:

// Atomic Values
simple_value(value) = value;

Composite values require more attention. An array, for instance, may be displayed using a horizontal alignment:

// Array
array(...) = frame(halign(...));

When GDB sends DDD the value of an array, the VSL function array() is invoked with array elements as values. A GDB array expression {1, 2, 3} is thus evaluated in VSL as

array(simple_value("1"), simple_value("2"), simple_value("3"))

which equals

"1" & "2" & "3"

a composite box holding a horizontal alignment of three string boxes. The actual VSL function used in DDD also puts delimiters between the elements and comes in a vertical variant as well.

Nested structures like multi-dimensional arrays are displayed by applying the array() function in a bottom-up fashion. First, array() is applied to the innermost structures; the resulting boxes are then passed as arguments to another array() invocation. The GDB output

{{"A", "B", "C"}, {"D", "E", "F"}}

representing a 2 * 3 array of character strings, is evaluated in VSL as

array(array("A", "B", "C"), array("A", "B", "C"))

resulting in a horizontal alignment of two more alignments representing the inner arrays.

Record structures are built in a similar manner, using a display function struct\_member rendering the record members. Names and values are separated by an equality sign:

// Member of a record structure
struct_member (name, value) =
  name & " = " & value;

The display function struct renders the record itself, using the valign() function.1

// Record structure
struct(...) = frame(valign(...));

This is a simple example; the actual VSL function used in DDD takes additional effort to align the equality signs; also, it ensures that language-specific delimiters are used, that collapsed structs are rendered properly, and so on.


Node:Writing Themes, Next:, Previous:Creating Displays, Up:Top

Writing Themes

The basic idea of a theme is to customize one or more aspects of the visual appearance of data. This is done by modifying specific VSL definitions.


Node:Display Title Color, Next:, Up:Writing Themes

Example: Changing the Display Title Color

As a simple example, consider the following task: You want to display display titles in blue instead of black. The VSL function which handles the colors of display titles is called title_color (see Displaying Colors). It is defined as

title_color(box) = color(box, "black");

All you'd have to do to change the color is to provide a new definition:

title_color(box) = color(box, "blue");

How do you do this? You create a data theme which modifies the definition.

Using your favourite text editor, you create a file named, say, blue-title.vsl in the directory ~/.ddd/themes/.

The file blue-title.vsl has the following content:

#pragma replace title_color
title_color(box) = color(box, "blue");

In DDD, select Data => Themes. You will find blue-title.vsl in a line on its own. Set the checkbox next to blue-title.vsl in order to activate it. Whoa! All display titles will now appear in blue.


Node:General Scheme, Next:, Previous:Display Title Color, Up:Writing Themes

The General Scheme

The general scheme for writing a theme is:


Node:Overriding vs. Replacing, Next:, Previous:General Scheme, Up:Writing Themes

Overriding vs. Replacing

In certain cases, you may not want to replace the original definition by your own, but rather extend the original definition.

As an example, consider the value_box function (see Displaying Data Displays). It is applied to every single value displayed. By default, it does nothing. So we could write a theme that leaves a little white space around values:

#pragma replace value_box
value_box(box) -> whiteframe(box);

or another theme that changes the color to black on yellow:

#pragma replace value_box
value_box(box) -> color(box, "black", "yellow");

However, we cannot apply both themes at once (say, to create a green-on-yellow scheme). This is because each of the two themes replaces the previous definition--the theme that comes last wins.

The solution to this problem is to set up the theme in such a way that it extends the original definition rather than to replace it. To do so, VSL provides an alternative to #pragma replace, namely #pragma override (see VSL Overriding Functions).

Like #pragma replace, the #pragma override declaration allows for a new definition of a function. In contrast to #pragma replace, though, uses of the function prior to #pragma override are not affected--they still refer to the old definition.

Here's a better theme that changes the color to black on yellow. First, it makes the old definition of value_box accessible as old_value_box. Then, it provides a new definition for value_box which refers to the old definition, saved in old_value_box.

#pragma override old_value_box
old_value_box(...) = value_box(...);

#pragma override value_box
value_box(value) -> color(old_value_box(value),
                          "black", "yellow");

Why do we need a #pragma override for old_value_box, too? Simple: to avoid name clashes between multiple themes. VSL has no scopes or name spaces for definitions, so we must resort to this crude, but effective scheme.


Node:A Complex Example, Next:, Previous:Overriding vs. Replacing, Up:Writing Themes

A Complex Example

As a more complex example, we define a theme that highlights all null pointers. First, we need a predicate is_null that tells us whether a pointer value is null:

// True if S1 ends in S2
ends_in(s1, s2) =
    let s1c = chars(s1),
        s2c = chars(s2) in suffix(s2c, s1c);

// True if null value
is_null(value) =
    (ends_in(value, "0x0") or ends_in(value, "nil"));

The null_pointer function tells us how we actually want to render null values:

// Rendering of null values
null_pointer(value) -> color(value, "red");

Now we go and redefine the pointer_value function such that null_pointer is applied only to null values:

#pragma override old_pointer_value
old_pointer_value(...) -> pointer_value(...);

#pragma override pointer_value

// Ordinary pointers
pointer_value (value) ->
    old_pointer_value(v)
    where v = (if (is_null(value)) then
                   null_pointer(value)
               else
                   value
               fi);

All we need now is the same definition for dereferenced pointers (that is, overriding the dereferenced_pointer_value function), and here we go!


Node:Future Work, Previous:A Complex Example, Up:Writing Themes

Future Work

With the information in this manual, you should be able to set up your own themes. If you miss anything, please let us know: simply write to ddd@gnu.org.

If there is sufficient interest, DDD's data themes will be further extended. Among the most wanted features is the ability to access and parse debuggee data from within VSL functions; this would allow user-defined processing of debuggee data. Let us know if you're interested--and keep in touch!


Node:DDD VSL Functions, Next:, Previous:Writing Themes, Up:Top

DDD VSL Functions

This appendix describes how DDD invokes VSL functions to create data displays.

The functions in this section are predefined in the library ddd.vsl. They can be used and replaced by DDD themes.

Please note: Functions marked as Global VSL Function must be (re-)defined using -> instead of =. See VSL Function Definitions, for details.


Node:Displaying Fonts, Next:, Up:DDD VSL Functions

Displaying Fonts

These are the function DDD uses for rendering boxes in different fonts:

small_rm (box) VSL Function
small_bf (box) VSL Function
small_it (box) VSL Function
small_bi (box) VSL Function
Returns box in small roman / bold face / italic / bold italic font.

small_size () VSL Function
Default size for small fonts.2

tiny_rm (box) VSL Function
tiny_bf (box) VSL Function
tiny_it (box) VSL Function
tiny_bi (box) VSL Function
Returns box in tiny roman / bold face / italic / bold italic font.

tiny_size () VSL Function
Default size for tiny fonts.3

title_rm (box) VSL Function
title_bf (box) VSL Function
title_it (box) VSL Function
title_bi (box) VSL Function
Returns box (a display title) in roman / bold face / italic / bold italic font.

value_rm (box) VSL Function
value_bf (box) VSL Function
value_it (box) VSL Function
value_bi (box) VSL Function
Returns box (a display value) in roman / bold face / italic / bold italic font.


Node:Displaying Colors, Next:, Previous:Displaying Fonts, Up:DDD VSL Functions

Displaying Colors

display_color (box) VSL Function
Returns box in the color used for displays. Default definition is
display_color(box) = color(box, "black", "white");

title_color (box) VSL Function
Returns box in the color used for display titles. Default definition is
title_color(box) = color(box, "black");

disabled_color (box) VSL Function
Returns box in the color used for disabled displays. Default definition is
disabled_color(box) = color(box, "white", "grey50");

simple_color (box) VSL Function
Returns box in the color used for simple values. Default definition is
simple_color(box) = color(box, "black");

text_color (box) VSL Function
Returns box in the color used for multi-line texts. Default definition is
text_color(box) = color(box, "black");

pointer_color (box) VSL Function
Returns box in the color used for pointers. Default definition is
pointer_color(box) = color(box, "blue4");

struct_color (box) VSL Function
Returns box in the color used for structs. Default definition is
struct_color(box) = color(box, "black");

list_color (box) VSL Function
Returns box in the color used for lists. Default definition is
list_color(box) = color(box, "black");

array_color (box) VSL Function
Returns box in the color used for arrays. Default definition is
array_color(box) = color(box, "blue4");

reference_color (box) VSL Function
Returns box in the color used for references. Default definition is
reference_color(box) = color(box, "blue4");

changed_color (box) VSL Function
Returns box in the color used for changed values. Default definition is
changed_color(box) = color(box, "black", "#ffffcc");

shadow_color (box) VSL Function
Returns box in the color used for display shadows. Default definition is
shadow_color(box) = color(box, "grey");


Node:Displaying Shadows, Next:, Previous:Displaying Colors, Up:DDD VSL Functions

Displaying Shadows

shadow (box) VSL Function
Return box with a shadow around it.


Node:Displaying Data Displays, Next:, Previous:Displaying Shadows, Up:DDD VSL Functions

Displaying Data Displays

DDD uses these functions to create data displays.

title (display_number, name) Global VSL Function
title (name) Global VSL Function
Returns a box for the display title. If display_number (a string) is given, this is prepended to the title.

annotation (name) Global VSL Function
Returns a box for an edge annotation. This typically uses a tiny font.

disabled () Global VSL Function
Returns a box to be used as value for disabled displays.

none () Global VSL Function
Returns a box for "no value" (i.e. undefined values). Default: an empty string.

value_box (value) Global VSL Function
Returns value in a display box. Default: leave unchanged.

display_box (title, value) Global VSL Function
display_box (value) Global VSL Function
Returns the entire display box. title comes from title(), value from value_box().


Node:Displaying Simple Values, Next:, Previous:Displaying Data Displays, Up:DDD VSL Functions

Displaying Simple Values

DDD uses these functions to display simple values.

simple_value (value) Global VSL Function
Returns a box for a simple non-numeric value (characters, strings, constants, ...). This is typically aligned to the left.

numeric_value (value) Global VSL Function
Returns a box for a simple numeric value. This is typically aligned to the right.

collapsed_simple_value () Global VSL Function
Returns a box for a collapsed simple value.


Node:Displaying Pointers, Next:, Previous:Displaying Simple Values, Up:DDD VSL Functions

Displaying Pointers

DDD uses these functions to display pointers.

pointer_value (value) Global VSL Function
Returns a box for a pointer value.

dereferenced_pointer_value (value) Global VSL Function
Returns a box for a dereferenced pointer value.

collapsed_pointer_value () Global VSL Function
Returns a box for a collapsed pointer.


Node:Displaying References, Next:, Previous:Displaying Pointers, Up:DDD VSL Functions

Displaying References

DDD uses these functions to display references.

reference_value (value) Global VSL Function
Returns a box for a reference value.

collapsed_reference_value () Global VSL Function
Returns a box for a collapsed reference.


Node:Displaying Arrays, Next:, Previous:Displaying References, Up:DDD VSL Functions

Displaying Arrays

DDD uses these functions to display arrays.

horizontal_array (values...) Global VSL Function
Returns a box for a horizontal array containing values.

vertical_array (values...) Global VSL Function
Returns a box for a vertical array containing values.

empty_array () Global VSL Function
Returns a box for an empty array.

collapsed_array () Global VSL Function
Returns a box for a collapsed array.

twodim_array (rows...) Global VSL Function
Returns a box for a two-dimensional array. Argument is a list of rows, suitable for use with tab() or dtab().

twodim_array_elem (value) Global VSL Function
Returns a box for an element in a two-dimensional array.


Node:Displaying Structs, Next:, Previous:Displaying Arrays, Up:DDD VSL Functions

Displaying Structs

A struct is a set of (name, value) pairs, and is also called "record" or "object". DDD uses these functions to display structs.

struct_value (members...) Global VSL Function
Returns a box for a struct containing members.

collapsed_struct_value () Global VSL Function
Returns a box for a collapsed struct.

empty_struct_value () Global VSL Function
Returns a box for an empty struct.

struct_member_name (name) Global VSL Function
Returns a box for a member name.

struct_member (name, sep, value, name_width) Global VSL Function
Returns a box for a struct member. name is the member name, typeset with struct_member_name(), sep is the separator (as determined by the current programming language), value is the typeset member value, and name_width is the maximum width of all member names.

horizontal_unnamed_struct () Global VSL Function
vertical_unnamed_struct () Global VSL Function
Returns a box for a horizontal / vertical unnamed struct, where member names are suppressed.

struct_member (value) Global VSL Function
Returns a box for a struct member in a struct where member names are suppressed.


Node:Displaying Lists, Next:, Previous:Displaying Structs, Up:DDD VSL Functions

Displaying Lists

A list is a set of (name, value) pairs not defined by the specific programming language. DDD uses this format to display variable lists.

list_value (members...) Global VSL Function
Returns a box for a list containing members.

collapsed_list_value () Global VSL Function
Returns a box for a collapsed list.

empty_list_value () Global VSL Function
Returns a box for an empty list.

list_member_name (name) Global VSL Function
Returns a box for a member name.

list_member (name, sep, value, name_width) Global VSL Function
Returns a box for a list member. name is the member name, typeset with list_member_name(), sep is the separator (as determined by the current programming language), value is the typeset member value, and name_width is the maximum width of all member names.

horizontal_unnamed_list () Global VSL Function
vertical_unnamed_list () Global VSL Function
Returns a box for a horizontal / vertical unnamed list, where member names are suppressed.

list_member (value) Global VSL Function
Returns a box for a list member in a list where member names are suppressed.


Node:Displaying Sequences, Next:, Previous:Displaying Lists, Up:DDD VSL Functions

Displaying Sequences

Sequences are lists of arbitrary, unstructured values.

sequence_value (values...) Global VSL Function
Returns a box for a list of values.

collapsed_sequence_value () Global VSL Function
Returns a box for a collapsed sequence.


Node:Displaying Multi-Line Texts, Next:, Previous:Displaying Sequences, Up:DDD VSL Functions

Displaying Multi-Line Texts

DDD uses these functions to display multi-line texts, such as status displays.

text_value (lines...) Global VSL Function
Returns a box for a list of lines (typically in a vertical alignment).

collapsed_text_value () Global VSL Function
Returns a box for a collapsed text.


Node:Displaying Extra Properties, Previous:Displaying Multi-Line Texts, Up:DDD VSL Functions

Displaying Extra Properties

DDD uses these functions to display additional properties.

repeated_value (value, n) Global VSL Function
Returns a box for a value that is repeated n times. Note: n is a number, not a string.

changed_value (value) Global VSL Function
Returns a box for a value that has changed since the last display. Typically, this invokes changed_color(value).


Node:VSL Library, Next:, Previous:DDD VSL Functions, Up:Top

VSL Library

This appendix describes the VSL functions available in the standard VSL library.

Unless otherwise stated, all following functions are defined in std.vsl.

For DDD themes, std.vsl need not be included explicitly.


Node:Library Conventions, Next:, Up:VSL Library

Conventions

Throughout this document, we write a = (a1, a2) to refer to individual box sizes. a1 stands for the horizontal size of a, and a2 stands for the vertical size of a.


Node:Space Functions, Next:, Previous:Library Conventions, Up:VSL Library

Space Functions


Node:Empty Space, Next:, Up:Space Functions

Empty Space

fill () VSL Function
Returns an empty box of width 0 and height 0 which stretches in both horizontal and vertical directions.

hfill () VSL Function
Returns a box of height 0 which stretches horizontally.

vfill () VSL Function
Returns a box of width 0 which stretches vertically.


Node:Black Lines, Next:, Previous:Empty Space, Up:Space Functions

Black Lines

rule () VSL Function
Returns a black box of width 0 and height 0 which stretches in both horizontal and vertical directions.

hrule ([thickness]) VSL Function
Returns a black box of width 0 and height thickness which stretches horizontally. thickness defaults to rulethickness() (typically 1 pixel).

vrule ([thickness]) VSL Function
Returns a black box of width thickness and height 0 which stretches vertically. thickness defaults to rulethickness() (typically 1 pixel).

rulethickness () VSL Function
Returns the default thickness for black rules (default: 1).


Node:White Space, Next:, Previous:Black Lines, Up:Space Functions

White Space

hwhite ([thickness]) VSL Function
Returns a black box of width 0 and height thickness which stretches horizontally. thickness defaults to whitethickness() (typically 2 pixels).

vwhite ([thickness]) VSL Function
Returns a black box of width thickness and height 0 which stretches vertically. thickness defaults to whitethickness() (typically 2 pixels).

whitethickness () VSL Function
Returns the default thickness for white rules (default: 2).


Node:Controlling Stretch, Next:, Previous:White Space, Up:Space Functions

Controlling Stretch

hfix (a) VSL Function
Returns a box containing a, but not stretchable horizontally.

vfix (a) VSL Function
Returns a box containing a, but not stretchable vertically.

fix (a) VSL Function
Returns a box containing a, but not stretchable in either direction.


Node:Box Dimensions, Previous:Controlling Stretch, Up:Space Functions

Box Dimensions

hspace (a) VSL Function
If a = (a1, a2), create a square empty box with a size of (a1, a1).

vspace (a) VSL Function
If a = (a1, a2), create a square empty box with a size of (a2, a2).

square (a) VSL Function
If a = (a1, a2), create a square empty box with a size of max(a1, a2).

box (n, m) VSL Function
Returns a box of size (n, m).


Node:Composition Functions, Next:, Previous:Space Functions, Up:VSL Library

Composition Functions


Node:Horizontal Composition, Next:, Up:Composition Functions

Horizontal Composition

(&) (a, b) VSL Function
(&) (boxes...) VSL Function
halign (boxes...) VSL Function
Returns a horizontal alignment of a and b; a is placed left of b. Typically written in inline form a & b.

The alternative forms (available in function-call form only) return a horizontal left-to-right alignment of their arguments.

hralign (boxes...) VSL Function
Returns a right-to-left alignment of its arguments.


Node:Vertical Composition, Next:, Previous:Horizontal Composition, Up:Composition Functions

Vertical Composition

(|) (a, b) VSL Function
(|) (boxes...) VSL Function
valign (boxes...) VSL Function
Returns a vertical alignment of a and b; a is placed above b. Typically written in inline form a | b.

The alternative forms (available in function-call form only) return a vertical top-to-bottom alignment of their arguments.

vralign (boxes...) VSL Function
Returns a bottom-to-top alignment of its arguments.

vlist (sep, boxes...) VSL Function
Returns a top-to-bottom alignment of boxes, where any two boxes are separated by sep.


Node:Textual Composition, Next:, Previous:Vertical Composition, Up:Composition Functions

Textual Composition

(~) (a, b) VSL Function
(~) (boxes...) VSL Function
talign (boxes...) VSL Function
Returns a textual concatenation of a and b. b is placed in the lower right unused corner of a. Typically written in inline form a ~ b.

The alternative forms (available in function-call form only) return a textual concatenation of their arguments.

tralign (boxes...) VSL Function
Returns a textual right-to-left concatenation of its arguments.

tlist (sep, boxes...) VSL Function
Returns a textual left-to-right alignment of boxes, where any two boxes are separated by sep.

commalist (boxes...) VSL Function
Shorthand for tlist(", ", boxes...).

semicolonlist (boxes...) VSL Function
Shorthand for tlist("; ", boxes...).


Node:Overlays, Previous:Textual Composition, Up:Composition Functions

Overlays

(^) (a, b) VSL Function
(^) (boxes...) VSL Function
Returns an overlay of a and b. a and b are placed in the same rectangular area, which is the maximum size of a and b; first, a is drawn, then b. Typically written in inline form a ^ b.

The second form (available in function-call form only) returns an overlay of its arguments.


Node:Arithmetic Functions, Next:, Previous:Composition Functions, Up:VSL Library

Arithmetic Functions

(+) (a, b) VSL Function
(+) (boxes...) VSL Function
Returns the sum of a and b. If a = (a1, a2) and b = (b1, b2), then a + b = (a1 + a2, b1 + b2). Typically written in inline form a + b.

The second form (available in function-call form only) returns the sum of its arguments.

The special form +a is equivalent to a.

(-) (a, b) VSL Function
Returns the difference of a and b. If a = (a1, a2) and b = (b1, b2), then a - b = (a1 - a2, b1 - b2). Typically written in inline form a - b.

The special form -a is equivalent to 0-a.

(*) (a, b) VSL Function
(*) (boxes...) VSL Function
Returns the product of a and b. If a = (a1, a2) and b = (b1, b2), then a * b = (a1 * a2, b1 * b2). Typically written in inline form a * b.

The second form (available in function-call form only) returns the product of its arguments.

(/) (a, b) VSL Function
Returns the quotient of a and b. If a = (a1, a2) and b = (b1, b2), then a / b = (a1 / a2, b1 / b2). Typically written in inline form a / b.

(%) (a, b) VSL Function
Returns the remainder of a and b. If a = (a1, a2) and b = (b1, b2), then a % b = (a1 % a2, b1 % b2). Typically written in inline form a % b.


Node:Comparison Functions, Next:, Previous:Arithmetic Functions, Up:VSL Library

Comparison Functions

(=) (a, b) VSL Function
Returns true (1) if a = b, and false (0), otherwise. a = b holds if a and b have the same size, the same structure, and the same content. Typically written in inline form a / b.

(<>) (a, b) VSL Function
Returns false (0) if a = b, and true (1), otherwise. a = b holds if a and b have the same size, the same structure, and the same content. Typically written in inline form a / b.

(<) (a, b) VSL Function
If a = (a1, a2) and b = (b1, b2), then this function returns true (1) if a1 < b1 or a2 < b2 holds; false (0), otherwise. Typically written in inline form a < b.

(<=) (a, b) VSL Function
If a = (a1, a2) and b = (b1, b2), then this function returns true (1) if a1 <= b1 or a2 <= b2 holds; false (0), otherwise. Typically written in inline form a <= b.

(>) (a, b) VSL Function
If a = (a1, a2) and b = (b1, b2), then this function returns true (1) if a1 > b1 or a2 > b2 holds; false (0), otherwise. Typically written in inline form a > b.

(>=) (a, b) VSL Function
If a = (a1, a2) and b = (b1, b2), then this function returns true (1) if a1 >= b1 or a2 >= b2 holds; false (0), otherwise. Typically written in inline form a >= b.


Node:Maximum and Minimum Functions, Up:Comparison Functions

Maximum and Minimum Functions

max (b1, b2, ...) VSL Function
Returns the maximum of its arguments; that is, the one box b in its arguments for which b > b1, b > b2, ... holds.

min (b1, b2, ...) VSL Function
Returns the maximum of its arguments; that is, the one box b in its arguments for which b < b1, b < b2, ... holds.


Node:Negation Functions, Next:, Previous:Comparison Functions, Up:VSL Library

Negation Functions

(not) (a) VSL Function
Returns true (1) if a is false, and false (0), otherwise. Typically written in inline form not a.

See VSL Boolean Operators, for and and or.


Node:Frame Functions, Next:, Previous:Negation Functions, Up:VSL Library

Frame Functions

ruleframe (a[, thickness]) VSL Function
Returns a within a black rectangular frame of thickness thickness. thickness defaults to rulethickness() (typically 1 pixel).

whiteframe (a[, thickness]) VSL Function
Returns a within a white rectangular frame of thickness thickness. thickness defaults to whitethickness() (typically 2 pixels).

frame (a) VSL Function
Returns a within a rectangular frame. Equivalent to ruleframe(whiteframe(a).

doubleframe (a) VSL Function
Shortcut for frame(frame(a)).

thickframe (a) VSL Function
Shortcut for ruleframe(frame(a)).


Node:Alignment Functions, Next:, Previous:Frame Functions, Up:VSL Library

Alignment Functions


Node:Centering Functions, Next:, Up:Alignment Functions

Centering Functions

hcenter (a) VSL Function
Returns box a centered horizontally within a (vertical) alignment.

Example: In a | hcenter(b) | c, b is centered relatively to a and c.

vcenter (a) VSL Function
Returns box a centered vertically within a (horizontal) alignment.

Example: In a & vcenter(b) & c, b is centered relatively to a and c.

center (a) VSL Function
Returns box a centered vertically and horizontally within an alignment.

Example: In 100 ^ center(b), b is centered within a square of size 100.


Node:Flushing Functions, Previous:Centering Functions, Up:Alignment Functions

Flushing Functions

n_flush (box) VSL Function
s_flush (box) VSL Function
w_flush (box) VSL Function
e_flush (box) VSL Function
Within an alignment, Flushes box to the center of a side.

Example: In 100 ^ s_flush(b), b is centered on the bottom side of a square of size 100.

nw_flush (box) VSL Function
sw_flush (box) VSL Function
ne_flush (box) VSL Function
se_flush (box) VSL Function
Within an alignment, Flushes box to a corner.

Example: In 100 ^ se_flush(b), b is placed in the lower right corner of a square of size 100.


Node:Emphasis Functions, Next:, Previous:Alignment Functions, Up:VSL Library

Emphasis Functions

underline (a) VSL Function
Returns a with a line underneath.

overline (a) VSL Function
Returns a with a line above it.

crossline (a) VSL Function
Returns a with a horizontal line across it.

doublestrike (a) VSL Function
Returns a in "poor man's bold": it is drawn two times, displaced horizontally by one pixel.


Node:Indentation Functions, Next:, Previous:Emphasis Functions, Up:VSL Library

Indentation Functions

indent (box) VSL Function
Return a box where white space of width indentamount() is placed left of box.

indentamount () VSL Function
Indent amount to be used in indent(); defaults to " " (two spaces).


Node:String Functions, Next:, Previous:Indentation Functions, Up:VSL Library

String Functions

To retrieve the string from a composite box, use string():

string (box) VSL Function
Return the string (in left-to-right, top-to-bottom order) within box.

To convert numbers to strings, use num():

num (a [, \varbase]) VSL Function
For a square box a = (a1, a1), returns a string containing a textual representation of a1. base must be between 2 and 16; it defaults to 10. Example: num(25) => "25")

dec (a) VSL Function
oct (a) VSL Function
bin (a) VSL Function
hex (a) VSL Function
Shortcut for num(a, 10), num(a, 8), num(a, 2), num(a, 16), respectively.


Node:List Functions, Next:, Previous:String Functions, Up:VSL Library

List Functions

The functions in this section require inclusion of the library list.vsl.

For themes, list.vsl need not be included explicitly.


Node:Creating Lists, Next:, Up:List Functions

Creating Lists

(::) (list1, list2, ...) VSL Function
Return the concatenation of the given lists. Typically written in inline form: [1] :: [2] :: [3] => [1, 2, 3].

append (list, elem) VSL Function
Returns list with elem appended at the end: append([1, 2, 3], 4) => [1, 2, 3, 4]


Node:List Properties, Next:, Previous:Creating Lists, Up:List Functions

List Properties

isatom (x) VSL Function
Returns True (1) if x is an atom; False (0) if x is a list.

islist (x) VSL Function
Returns True (1) if x is a list; False (0) if x is an atom.

member (x, list) VSL Function
Returns True (1) if x is an element of list; False (0) if not: member(1, [1, 2, 3]) => true

prefix (sublist, list) VSL Function
suffix (sublist, list) VSL Function
sublist (sublist, list) VSL Function
Returns True (1) if sublist is a prefix / suffix / sublist of list; False (0) if not: prefix([1], [1, 2]) => true, suffix([3], [1, 2]) => false, sublist([2, 2], [1, 2, 2, 3]) => true,

length (list) VSL Functions
Returns the number of elements in list: length([1, 2, 3]) => 3


Node:Accessing List Elements, Next:, Previous:List Properties, Up:List Functions

Accessing List Elements

car (list) VSL Function
head (list) VSL Function
Returns the first element of list: car([1, 2, 3]) => 1

cdr (list) VSL Function
tail (list) VSL Function
Returns list without its first element: cdr([1, 2, 3]) => [2, 3]

elem (list, n) VSL Function
Returns the n-th element (starting with 0) of list: elem([4, 5, 6], 0) => 4

pos (elem, list) VSL Function
Returns the position of elem in list (starting with 0): pos(4, [1, 2, 4]) => 2

last (list) VSL Function
Returns the last element of list: last([4, 5, 6]) => 6


Node:Manipulating Lists, Next:, Previous:Accessing List Elements, Up:List Functions

Manipulating Lists

reverse (list) VSL Function
Returns a reversed list: reverse([3, 4, 5]) => [5, 4, 3]

delete (list, elem) VSL Function
Returns list, with all elements elem removed: delete([4, 5, 5, 6], 5) => [4, 6]

select (list, elem) VSL Function
Returns list, with the first element elem removed: select([4, 5, 5, 6], 5) => [4, 5, 6]

flat (list) VSL Function
Returns flattened list: flat([[3, 4], [[5], [6]]]) => [3, 4, 5, 6]

sort (list) VSL Function
Returns sortened list (according to box size): sort([7, 4, 9]) => [4, 7, 9]


Node:Lists and Strings, Previous:Manipulating Lists, Up:List Functions

Lists and Strings

chars (s) VSL Function
Returns a list of all characters in the box s: chars("abc") => ["a", "b", "c"]

list (list) VSL Function
Returns a string, pretty-printing the list: list([4, 5, 6]) => "[4, 5, 6]"


Node:Table Functions, Next:, Previous:List Functions, Up:VSL Library

Table Functions

The functions in this section require inclusion of the library tab.vsl.

For themes, tab.vsl need not be included explicitly.

tab (table) VSL Function
Return table (a list of lists) aligned in a table: tab([[1, 2, 3], [4, 5, 6], [7, 8]]) =>
1 2 3
4 5 6
7 8

dtab (table) VSL Function
Like tab, but place delimiters (horizontal and vertical rules) around table elements.

tab_elem (x) VSL Function
Returns padded table element x. Its default definition is:
tab_elem([]) = tab_elem(0);     // empty table
tab_elem(x)  = whiteframe(x);   // padding


Node:Font Functions, Next:, Previous:Table Functions, Up:VSL Library

Font Functions

The functions in this section require inclusion of the library fonts.vsl.

For themes, fonts.vsl need not be included explicitly.


Node:Font Basics, Next:, Up:Font Functions

Font Basics

font (box, font) VSL Function
Returns box, with all strings set in font (a valid X11 font description)


Node:Font Name Selection, Next:, Previous:Font Basics, Up:Font Functions

Font Name Selection

weight_bold () VSL Function
weight_medium () VSL Function
Font weight specifier in fontname() (see below).

slant_unslanted () VSL Function
slant_italic () VSL Function
Font slant Specifier in fontname() (see below).

family_times () VSL Function
family_courier () VSL Function
family_helvetica () VSL Function
family_new_century () VSL Function
family_typewriter () VSL Function
Font family specifier in fontname() (see below).

fontname ([weight, [slant, [family, [size]]]]) VSL Function
Returns a fontname, suitable for use with font().
  • weight defaults to stdfontweight() (see below).
  • slant defaults to stdfontslant() (see below).
  • family defaults to stdfontfamily() (see below).
  • size is a pair (pixels, points) where pixels being zero means to use points instead and vice versa. defaults to stdfontsize() (see below).


Node:Font Defaults, Next:, Previous:Font Name Selection, Up:Font Functions

Font Defaults

stdfontweight () VSL Function
Default font weight: weight_medium().

stdfontslant () VSL Function
Default font slant: slant_unslanted().

stdfontfamily () VSL Function
Default font family: family_times().

DDD replaces this as set in the DDD font preferences. Use ddd --fonts to see the actual definitions.

stdfontsize () VSL Function
Default font size: (stdfontpixels(), stdfontpoints()).

DDD replaces this as set in the DDD font preferences. Use ddd --fonts to see the actual definitions.

stdfontpixels () VSL Function
Default font size (in pixels): 0, meaning to use stdfontpoints() instead.

stdfontpoints () VSL Function
Default font size (in 1/10 points): 120.


Node:Font Selection, Previous:Font Defaults, Up:Font Functions

Font Selection

rm (box [, family [, size]]) VSL Function
bf (box [, family [, size]]) VSL Function
it (box [, family [, size]]) VSL Function
bi (box [, family [, size]]) VSL Function
Returns box in roman / bold face / italic / bold italic. family specifies one of the font families; it defaults to stdfontfamily() (see above). size specifies a font size; it defaults to stdfontsize() (see above).


Node:Color Functions, Next:, Previous:Font Functions, Up:VSL Library

Color Functions

The functions in this section require inclusion of the library colors.vsl.

For themes, colors.vsl need not be included explicitly.

color (box, foreground [, background]]) VSL Function
Returns box, where the foreground color will be drawn using the foreground color. If background is specified as well, it will be used for drawing the background. Both foreground and background are strings specifying a valid X11 color.


Node:Arc Functions, Next:, Previous:Color Functions, Up:VSL Library

Arc Functions

The functions in this section require inclusion of the library arcs.vsl.

For themes, arcs.vsl must be included explicitly, using a line

#include <arcs.vsl>
at the beginning of the theme.


Node:Arc Basics, Next:, Up:Arc Functions

Arc Basics

arc (start, length [, thickness]) VSL Function
Returns a stretchable box with an arc of length, starting at angle start. start and length must be multiples of 90 (degrees). The angle of start is specified clockwise relative to the 9 o'clock position. thickness defaults to arcthickness() (see below).

arcthickness () VSL Function
Default width of arcs. Defaults to rulethickness().


Node:Custom Arc Functions, Previous:Arc Basics, Up:Arc Functions

Custom Arc Functions

oval (box) VSL Function
Returns an oval containing box. Example: oval("33").

ellipse (box) VSL Function
ellipse () VSL Function
Returns an ellipse containing box. Example: ellipse("START"). If box is omitted, the ellipse is stretchable and expands to the available space.

circle (box) VSL Function
Returns a circle containing box. Example: circle(10).


Node:Slope Functions, Previous:Arc Functions, Up:VSL Library

Slope Functions

The functions in this section require inclusion of the library slopes.vsl.

For themes, slopes.vsl must be included explicitly, using a line

#include <slopes.vsl>
at the beginning of the theme.


Node:Slope Basics, Next:, Up:Slope Functions

Slope Basics

rise ([thickness]) VSL Function
Create a stretchable box with a line from the lower left to the upper right corner. thickness defaults to slopethickness() (see below).

fall ([thickness]) VSL Function
Create a stretchable box with a line from the upper left to the lower right corner. thickness defaults to slopethickness() (see below).

slopethickness () VSL Function
Default thickness of slopes. Defaults to rulethickness().


Node:Arrow Functions, Next:, Previous:Slope Basics, Up:Slope Functions

Arrow Functions

n_arrow () VSL Function
w_arrow () VSL Function
s_arrow () VSL Function
e_arrow () VSL Function
Returns a box with an arrow pointing to the upper, left, lower, or right side, respectively.

nw_arrow () VSL Function
ne_arrow () VSL Function
sw_arrow () VSL Function
se_arrow () VSL Function
Returns a box with an arrow pointing to the upper left, upper right, lower left, or lower right side, respectively.


Node:Custom Slope Functions, Previous:Arrow Functions, Up:Slope Functions

Custom Slope Functions

punchcard (box) VSL Function
Returns a punchcard containing box.

rhomb (box) VSL Function
Returns a rhomb containing box.

octogon (box) VSL Function
Returns an octogon containing box.


Node:VSL Reference, Next:, Previous:VSL Library, Up:Top

VSL Reference

This appendix describes the VSL language.


Node:VSL Boxes, Next:, Up:VSL Reference

Boxes

VSL knows two data types. The most common data type is the box. A box is a rectangular area with a content, a size, and a stretchability.

Boxes are either atomic or composite. A composite box is built from two or more other boxes. These boxes can be aligned horizontally, vertically, or otherwise.

Boxes have a specific minimum size, depending on their content. We say `minimum' size here, because some boxes are stretchable--that is, they can fill up the available space.

If you have a vertical alignment of three boxes A, B, and C, like this:

AAAAAA
AAAAAA
  B
  B
CCCCCC
CCCCCC
and B is stretchable horizontally, then B will fill up the available horizontal space:
AAAAAA
AAAAAA
BBBBBB
BBBBBB
CCCCCC
CCCCCC

If two or more boxes compete for the same space, the space will be distributed in proportion to their stretchability.

An atomic stretchable box has a stretchability of 1. An alignment of multiple boxes stretchable in the direction of the alignment boxes will have a stretchability which is the sum of all stretchabilities.

If you have a vertical alignment of three boxes A, B, C, D, and E, like this:

AAAAAA
AAAAAA
BC   D
BC   D
EEEEEE
EEEEEE
and B, C, and D are stretchable horizontally (with a stretchability of 1), then the horizontal alignment of B and C will have a stretchability of 2. Thus, the alignment of B and C gets two thirds of the available space; D gets the remaining third.
AAAAAA
AAAAAA
BBCCDD
BBCCDD
EEEEEE
EEEEEE


Node:VSL Lists, Next:, Previous:VSL Boxes, Up:VSL Reference

Lists

Besides boxes, VSL knows lists. A list is not a box--it has no size or stretchability. A list is a simple means to structure data.

VSL lists are very much like lists in functional languages like Lisp or Scheme. They consist of a head (typically a list element) and a tail (which is either a list remainder or the empty list).


Node:VSL Expressions, Next:, Previous:VSL Lists, Up:VSL Reference

Expressions


Node:VSL String Literals, Next:, Up:VSL Expressions

String Literals

The expression "text" returns a box containing text. text is parsed according to C syntax rules.

Multiple string expressions may follow each other to form a larger constant, as in C++. "text1" "text2" is equivalent to "text1text2"

Strings are not stretchable.


Node:VSL Number Literals, Next:, Previous:VSL String Literals, Up:VSL Expressions

Number Literals

Any constant integer n evaluates to a number--that is, a non-stretchable empty square box with size (n, n).


Node:VSL List Literals, Next:, Previous:VSL Number Literals, Up:VSL Expressions

List Literals

The expression [a, b, ...] evaluates to a list containing the element a, b, .... [] is the empty list.

The expression [head : tail] evaluates to a list whose first element is head and whose remainder (typically a list) is tail.

In most contexts, round parentheses can be used as alternatives to square brackets. Thus, (a, b) is a list with two elements, and () is the empty list.

Within an expression, though, square parentheses must be used to create a list with one element. In an expression, the form (a) is not a list, but an alternative notation for a.


Node:VSL Conditionals, Next:, Previous:VSL List Literals, Up:VSL Expressions

Conditionals

A box a = (a1, a2) is called true if a1 or a2 is non-zero. It is called false if both a1 or a2 are zero.

The special form

if a then b else c fi
returns b if a is true, and c otherwise. Only one of b or c is evaluated.

The special form

elsif a2 then b2 else c fi
is equivalent to
else if a2 then b2 else c fi fi


Node:VSL Boolean Operators, Next:, Previous:VSL Conditionals, Up:VSL Expressions

Boolean Operators

The special form

a and b
is equivalent to
if a then b else 0 fi

The special form

a or b
is equivalent to
if a then 1 else b fi

The special form

not a
is equivalent to
if a then 0 else 1 fi

Actually, not is realized as a function; See Negation Functions, for details.


Node:VSL Local Variables, Next:, Previous:VSL Boolean Operators, Up:VSL Expressions

Local Variables

You can introduce local variables using let and where:

let v1 = e1 in e
makes v1 available as replacement for e1 in the expression e.

Example:

let pi = 3.1415 in 2 * pi => 6.2830

The special form

let v1 = e1, v2 = e2, ... in e
is equivalent to
let v1 = e1 in let v2 = e2 in let ... in e

As an alternative, you can also use the where form:

e where v1 = e1
is equivalent to
let v1 = e1 in e

Example:

("here lies" | name) where
    name = ("one whose name" | "was writ in water")

The special form

e where v1 = e1, v2 = e2, ...
is equivalent to
let v1 = e1, v2 = e2, ... in e


Node:VSL Let Patterns, Previous:VSL Local Variables, Up:VSL Expressions

Let Patterns

You can access the individual elements of a list or some composite box by giving an appropriate pattern:

let (left, right) = pair in expr

If pair has the value, say, (3, 4), then left will be available as a replacement for 3, and right will be available as a replacement for 4 in expr.

A special pattern is available for accessing the head and the tail of a list:

let [head : tail] = list in expr

If expr has the value, say, [3, 4, 5], then head will be 3, and tail will be [4, 5] in expr.


Node:VSL Function Calls, Next:, Previous:VSL Expressions, Up:VSL Reference

Function Calls

A function call takes the form

name list

which invokes the (previously declared or defined) function with an argument of list. Normally, list is a list literal (see VSL List Literals) written with round brackets.


Node:VSL Constant Definitions, Next:, Previous:VSL Function Calls, Up:VSL Reference

Constant Definitions

A VSL file consists of a list of definitions.

A constant definition takes the form

name = expression;

Any later definitions can use name as a replacement for expression.

Example:

true = 1;
false = 0;


Node:VSL Function Definitions, Next:, Previous:VSL Constant Definitions, Up:VSL Reference

Function Definitions

In VSL, all functions either map a list to a box or a list to a list. A function definition takes the form

name list = expression;

where list is a list literal (see VSL List Literals).

The list literal is typically written in round parentheses, making the above form look like this:

name(param1, param2, ...) = expression;

The = is replaced by -> if name is a global definition--that is, name can be called from a library client such as DDD. A local definition (with =) can be called only from other VSL functions.4


Node:VSL Function Parameters, Next:, Up:VSL Function Definitions

Function Parameters

The parameter list list may contain names of formal parameters. Upon a function call, these are bound to the actual arguments.

If the function

sum(a, b) = a + b;
is called as
sum(2. 3)
then a will be bound to 2 and b will be bound to 3, evaluating to 5.


Node:VSL Unused Parameters, Up:VSL Function Parameters

VSL Unused Parameters

Unused parameters cause a warning, as in this example:

first_arg(a, dummy) = a;        // Warning

If a parameter has the name _, it will not be bound to the actual argument (and can thus not be used). Use _ as parameter name for unused arguments:

first_arg(a, _) = a;            // No warning

_ can be used multiple times in a parameter list.


Node:VSL Function Patterns, Next:, Previous:VSL Function Parameters, Up:VSL Function Definitions

Function Patterns

A VSL function may have multiple definitions, each with a specific pattern. The first definition whose pattern matches the actual argument is used.

What does `matching' mean? Within a pattern,

Here are some examples. The num() function (see String Functions) can take either one or two arguments. The one-argument definition simply invokes the two-argument definition:

num(a, base) = ...;
num(a) = num(a, 10);

Here's another example: The digit function returns a string representation for a single number. It has multiple definitions, all dependent on the actual argument:

digit(0) = "0";
digit(1) = "1";
digit(2) = "2";
digit(3) = "3";
digit(4) = "4";
digit(5) = "5";
digit(6) = "6";
digit(7) = "7";
digit(8) = "8";
digit(9) = "9";
digit(10) = "a";
digit(11) = "b";
digit(12) = "c";
digit(13) = "d";
digit(14) = "e";
digit(15) = "f";
digit(_) = fail("invalid digit() argument");

Formal parameters ending in ... are useful for defining aliases of functions. The definition

roman(...) = rm(...);
makes roman an alias of rm--any parameters (regardless how many) passed to roman will be passed to rm.

Here's an example of how formal parameters ending in ... can be used to realize variadic functions, taking any number of arguments (see Maximum and Minimum Functions):

max(a) = a;
max(a, b, ...) = if a > b then max(a, ...) else max(b, ...) fi;
min(a) = a;
min(a, b, ...) = if a < b then min(a, ...) else min(b, ...) fi;


Node:VSL Declaring Functions, Next:, Previous:VSL Function Patterns, Up:VSL Function Definitions

Declaring Functions

If you want to use a function before it has been defined, just write down its signature without specifying a body. Here's an example:

num(a, base);                   // declaration
num(a) = num(a, 10);

Remember to give a definition later on, though.


Node:VSL Redefining Functions, Next:, Previous:VSL Declaring Functions, Up:VSL Function Definitions

Redefining Functions

You can redefine a VSL function even after its original definition. You can


Node:VSL Replacing Functions, Next:, Previous:VSL Redefining Functions, Up:VSL Function Definitions

Replacing Functions

To remove an original definition, use

#pragma replace name

This removes all previous definitions of name. Be sure to provide your own definitions, though.

#pragma replace is typically used to change defaults:

#include "fonts.vsl"            // defines stdfontsize()

#pragma replace stdfontsize()   // replace def
stdfontsize() = 20;

All existing function calls will now refer to the new definition.


Node:VSL Overriding Functions, Previous:VSL Replacing Functions, Up:VSL Function Definitions

Overriding Functions

To override an original definition, use

#pragma override name

This makes all later definitions use your new definition of name. Earlier definitions, however, still refer to the old definition.

#pragma override is typically used if you want to redefine a function while still refering to the old definition:

#include "fonts.vsl"            // defines stdfontsize()

// Save old definition
old_stdfontsize() = stdfontsize();

#pragma override stdfontsize()

// Refer to old definition
stdfontsize() = old_stdfontsize() * 2;

Since we used #pragma override, we can use old_stdfontsize() to refer to the original definition of stdfontsize().


Node:VSL Includes, Next:, Previous:VSL Function Definitions, Up:VSL Reference

Includes

In a VSL file, you can include at any part the contents of another VSL file, using one of the special forms

#include "file"
#include <file>

The form <file> looks for VSL files in a number of standard directories; the form "file" first looks in the directory where the current file resides.

Any included file is included only once.

In DDD, you can set these places using the vslPath resource. See Customizing Display Appearance, for details.


Node:VSL Operators, Next:, Previous:VSL Includes, Up:VSL Reference

Operators

VSL comes with a number of inline operators, which can be used to compose boxes. With raising precedence, these are:

or
and
= <>
<= < >= >
::
|
^
~
&
+ -
* / %
not

Except for or and and, these operators are mapped to function calls. Each invocation of an operator @ in the form a @ b gets translated to a call of the VSL function with the special name (@). This VSL function can be defined just like any other VSL function.

For instance, the expression a + b gets translated to a function call (+)(a, b); a & b invokes (&)(a, b).

In the file builtin.vsl, you can actually find definitions of these functions:

(&)(...) = __op_halign(...);
(+)(...) = __op_plus(...);

The functions __op_halign and __op_plus are the names by which the (&) and (+) functions are implemented. In this document, though, we will not look further at these internals.

Here are the places where the operator functions are described:


Node:VSL Syntax Summary, Previous:VSL Operators, Up:VSL Reference

Syntax Summary

The following file summarizes the syntax of VSL files.

/*** VSL file ***/

file                    :       item_list

item_list               :       /* empty */
                        |       item_list item

item                    :       function_declaration ';'
                        |       function_definition ';'
                        |       override_declaration
                        |       replace_declaration
                        |       include_declaration
                        |       line_declaration
                        |       ';'
                        |       error ';'

/*** functions ***/

function_declaration    :       function_header

function_header         :       function_identifier function_argument
                        |       function_identifier

function_identifier     :       identifier
                        |       '(' '==' ')'
                        |       '(' '<>' ')'
                        |       '(' '>' ')'
                        |       '(' '>=' ')'
                        |       '(' '<' ')'
                        |       '(' '<=' ')'
                        |       '(' '&' ')'
                        |       '(' '|' ')'
                        |       '(' '^' ')'
                        |       '(' '~' ')'
                        |       '(' '+' ')'
                        |       '(' '-' ')'
                        |       '(' '*' ')'
                        |       '(' '/' ')'
                        |       '(' '%' ')'
                        |       '(' '::' ')'
                        |       '(' 'not' ')'

identifier              :       IDENTIFIER

function_definition     :       local_definition
                        |       global_definition

local_definition        :       local_header function_body

local_header            :       function_header '='

global_definition       :       global_header function_body

global_header           :       function_header '->'

function_body           :       box_expression_with_defs



/*** expressions ***/

/*** let, where ***/

box_expression_with_defs:       box_expression_with_wheres
                        |       'let' var_definition in_box_expression

in_box_expression       :       'in' box_expression_with_defs
                        |       ',' var_definition in_box_expression

box_expression_with_wheres:     box_expression
                        |       box_expression_with_where

box_expression_with_where:      box_expression_with_wheres
                                'where' var_definition
                        |       box_expression_with_where
                                ',' var_definition

var_definition          :       box_expression '=' box_expression


/*** basic expressions ***/

box_expression          :       '(' box_expression_with_defs ')'
                        |       list_expression
                        |       const_expression
                        |       binary_expression
                        |       unary_expression
                        |       cond_expression
                        |       function_call
                        |       argument_or_function

list_expression         :       '[' ']'
                        |       '[' box_expression_list ']'
                        |       '(' ')'
                        |       '(' multiple_box_expression_list ')'

box_expression_list     :       box_expression_with_defs
                        |       multiple_box_expression_list

multiple_box_expression_list:   box_expression ':' box_expression
                        |       box_expression ',' box_expression_list
                        |       box_expression '...'
                        |       '...'

const_expression        :       string_constant
                        |       numeric_constant

string_constant         :       STRING
                        |       string_constant STRING

numeric_constant        :       INTEGER

function_call           :       function_identifier function_argument

unary_expression        :       'not' box_expression
                        |       '+' box_expression
                        |       '-' box_expression


/*** operators ***/

binary_expression       :       box_expression '=' box_expression
                        |       box_expression '<>' box_expression
                        |       box_expression '>' box_expression
                        |       box_expression '>=' box_expression
                        |       box_expression '<' box_expression
                        |       box_expression '<=' box_expression
                        |       box_expression '&' box_expression
                        |       box_expression '|' box_expression
                        |       box_expression '^' box_expression
                        |       box_expression '~' box_expression
                        |       box_expression '+' box_expression
                        |       box_expression '-' box_expression
                        |       box_expression '*' box_expression
                        |       box_expression '/' box_expression
                        |       box_expression '%' box_expression
                        |       box_expression '::' box_expression
                        |       box_expression 'or' box_expression
                        |       box_expression 'and' box_expression

cond_expression         :       'if' box_expression
                                'then' box_expression_with_defs
                                else_expression
                                'fi'

else_expression         :       'elsif' box_expression
                                'then' box_expression_with_defs
                                else_expression
                        |       'else' box_expression_with_defs

function_argument       :       list_expression
                        |       '(' box_expression_with_defs ')'

argument_or_function    :       identifier


/*** directives ***/

override_declaration    :       '#pragma' 'override' override_list

override_list           :       override_identifier
                        |       override_list ',' override_identifier

override_identifier     :       function_identifier

replace_declaration     :       '#pragma' 'replace' replace_list

replace_list            :       replace_identifier
                        |       replace_list ',' replace_identifier

replace_identifier      :       function_identifier

include_declaration     :       '#include' '"' SIMPLE_STRING '"'
                        |       '#include' '<' SIMPLE_STRING '>'

line_declaration        :       '#line' INTEGER
                        |       '#line' INTEGER STRING


Node:Documentation License, Next:, Previous:VSL Reference, Up:Top

GNU Free Documentation License

Version 1.1, March 2000

Copyright (C) 2000  Free Software Foundation, Inc.
59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

  1. PREAMBLE

    The purpose of this License is to make a manual, textbook, or other written document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

    This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

    We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

  2. APPLICABILITY AND DEFINITIONS

    This License applies to any manual or other work that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as "you".

    A "Modified Version" of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

    A "Secondary Section" is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (For example, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.

    The "Invariant Sections" are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License.

    The "Cover Texts" are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License.

    A "Transparent" copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, whose contents can be viewed and edited directly and straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup has been designed to thwart or discourage subsequent modification by readers is not Transparent. A copy that is not "Transparent" is called "Opaque".

    Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML designed for human modification. Opaque formats include PostScript, PDF, proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML produced by some word processors for output purposes only.

    The "Title Page" means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, "Title Page" means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.

  3. VERBATIM COPYING

    You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.

    You may also lend copies, under the same conditions stated above, and you may publicly display copies.

  4. COPYING IN QUANTITY

    If you publish printed copies of the Document numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

    If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

    If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a publicly-accessible computer-network location containing a complete Transparent copy of the Document, free of added material, which the general network-using public has access to download anonymously at no charge using public-standard network protocols. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

    It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.

  5. MODIFICATIONS

    You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

    A. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
    B. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has less than five).
    C. State on the Title page the name of the publisher of the Modified Version, as the publisher.
    D. Preserve all the copyright notices of the Document.
    E. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
    F. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
    G. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice.
    H. Include an unaltered copy of this License.
    I. Preserve the section entitled "History", and its title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section entitled "History" in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
    J. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the "History" section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
    K. In any section entitled "Acknowledgements" or "Dedications", preserve the section's title, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
    L. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
    M. Delete any section entitled "Endorsements". Such a section may not be included in the Modified Version.
    N. Do not retitle any existing section as "Endorsements" or to conflict in title with any Invariant Section.

    If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.

    You may add a section entitled "Endorsements", provided it contains nothing but endorsements of your Modified Version by various parties-for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

    You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

    The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

  6. COMBINING DOCUMENTS

    You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice.

    The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

    In the combination, you must combine any sections entitled "History" in the various original documents, forming one section entitled "History"; likewise combine any sections entitled "Acknowledgements", and any sections entitled "Dedications". You must delete all sections entitled "Endorsements."

  7. COLLECTIONS OF DOCUMENTS

    You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

    You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

  8. AGGREGATION WITH INDEPENDENT WORKS

    A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, does not as a whole count as a Modified Version of the Document, provided no compilation copyright is claimed for the compilation. Such a compilation is called an "aggregate", and this License does not apply to the other self-contained works thus compiled with the Document, on account of their being thus compiled, if they are not themselves derivative works of the Document.

    If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one quarter of the entire aggregate, the Document's Cover Texts may be placed on covers that surround only the Document within the aggregate. Otherwise they must appear on covers around the whole aggregate.

  9. TRANSLATION

    Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License provided that you also include the original English version of this License. In case of a disagreement between the translation and the original English version of this License, the original English version will prevail.

  10. TERMINATION

    You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.

  11. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

  Copyright (C)  year  your name.
  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.1
  or any later version published by the Free Software Foundation;
  with the Invariant Sections being list their titles, with the
  Front-Cover Texts being list, and with the Back-Cover Texts being list.
  A copy of the license is included in the section entitled ``GNU
  Free Documentation License''.
If you have no Invariant Sections, write "with no Invariant Sections" instead of saying which ones are invariant. If you have no Front-Cover Texts, write "no Front-Cover Texts" instead of "Front-Cover Texts being list"; likewise for Back-Cover Texts.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.


Node:Index, Previous:Documentation License, Up:Top

Index


Footnotes

  1. valign() is similar to halign(), but builds a vertical alignment.

  2. DDD replaces this as set in the DDD font preferences. Use ddd --fonts to see the actual definitions.

  3. DDD replaces this as set in the DDD font preferences. Use ddd --fonts to see the actual definitions.

  4. The distinction into global and local definitions is useful when optimizing the library: local definitions that are unused within the library can be removed, while global definitions cannot.