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 3.3.12.
Copyright © 2001 Universität Passau
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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 of DDD.)
We begin with a short discussion of how DDD actually creates displays from data.
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.
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.
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.
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.
The general scheme for writing a theme is:
Find out which VSL function function is responsible for a specific task. See DDD VSL Functions, for details on the VSL functions used by DDD.
Write a theme (a text file) with the following content:
#pragma replace function
function(args) = definition;
This will replace the existing definition of function by your new definition definition. It is composed of two parts:
Please note: If the function function is marked as ‘Global VSL Function’, it must be (re-)defined using ‘->’ instead of ‘=’; See VSL Function Definitions, for details. You may also want to consider ‘#pragma override’ instead; See Overriding vs. Replacing, for details.
For your personal use, this is normally the directory ~/.ddd/themes/.
Besides your personal directory, DDD also searches for themes in its theme directory, typically /usr/local/share/ddd-3.3.12/themes/.
The DDD ‘vslPath’ resource controls the actual path where DDD looks for themes. See VSL Resources, for details.
You're done!
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.
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!
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!
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.
These are the function DDD uses for rendering boxes in different fonts:
Returns box in small roman / bold face / italic / bold italic font.
Returns box in tiny roman / bold face / italic / bold italic font.
Returns box (a display title) in roman / bold face / italic / bold italic font.
Returns box (a display value) in roman / bold face / italic / bold italic font.
Returns box in the color used for displays. Default definition is
display_color(box) = color(box, "black", "white");
Returns box in the color used for display titles. Default definition is
title_color(box) = color(box, "black");
Returns box in the color used for disabled displays. Default definition is
disabled_color(box) = color(box, "white", "grey50");
Returns box in the color used for simple values. Default definition is
simple_color(box) = color(box, "black");
Returns box in the color used for multi-line texts. Default definition is
text_color(box) = color(box, "black");
Returns box in the color used for pointers. Default definition is
pointer_color(box) = color(box, "blue4");
Returns box in the color used for structs. Default definition is
struct_color(box) = color(box, "black");
Returns box in the color used for lists. Default definition is
list_color(box) = color(box, "black");
Returns box in the color used for arrays. Default definition is
array_color(box) = color(box, "blue4");
Returns box in the color used for references. Default definition is
reference_color(box) = color(box, "blue4");
Returns box in the color used for changed values. Default definition is
changed_color(box) = color(box, "black", "#ffffcc");
Returns box in the color used for display shadows. Default definition is
shadow_color(box) = color(box, "grey");
DDD uses these functions to create data displays.
Returns a box for the display title. If display_number (a string) is given, this is prepended to the title.
Returns a box for an edge annotation. This typically uses a tiny font.
Returns a box for “no value” (i.e. undefined values). Default: an empty string.
Returns the entire display box. title comes from
title(), value fromvalue_box().
DDD uses these functions to display simple values.
Returns a box for a simple non-numeric value (characters, strings, constants, ...). This is typically aligned to the left.
Returns a box for a simple numeric value. This is typically aligned to the right.
DDD uses these functions to display pointers.
Returns a box for a dereferenced pointer value.
DDD uses these functions to display references.
DDD uses these functions to display arrays.
Returns a box for a horizontal array containing values.
Returns a box for a vertical array containing values.
Returns a box for a two-dimensional array. Argument is a list of rows, suitable for use with
tab()ordtab().
Returns a box for an element in a two-dimensional array.
A struct is a set of (name, value) pairs, and is also called “record” or “object”. DDD uses these functions to display structs.
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.
Returns a box for a horizontal / vertical unnamed struct, where member names are suppressed.
Returns a box for a struct member in a struct where member names are suppressed.
A list is a set of (name, value) pairs not defined by the specific programming language. DDD uses this format to display variable lists.
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.
Returns a box for a horizontal / vertical unnamed list, where member names are suppressed.
Returns a box for a list member in a list where member names are suppressed.
Sequences are lists of arbitrary, unstructured values.
DDD uses these functions to display multi-line texts, such as status displays.
Returns a box for a list of lines (typically in a vertical alignment).
DDD uses these functions to display additional properties.
Returns a box for a value that is repeated n times. Note: n is a number, not a string.
Returns a box for a value that has changed since the last display. Typically, this invokes
changed_color(value).
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.
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.
Returns an empty box of width 0 and height 0 which stretches in both horizontal and vertical directions.
Returns a black box of width 0 and height 0 which stretches in both horizontal and vertical directions.
Returns a black box of width 0 and height thickness which stretches horizontally. thickness defaults to
rulethickness()(typically 1 pixel).
Returns a black box of width thickness and height 0 which stretches vertically. thickness defaults to
rulethickness()(typically 1 pixel).
Returns a black box of width 0 and height thickness which stretches horizontally. thickness defaults to
whitethickness()(typically 2 pixels).
Returns a black box of width thickness and height 0 which stretches vertically. thickness defaults to
whitethickness()(typically 2 pixels).
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.
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.
Returns a top-to-bottom alignment of boxes, where any two boxes are separated by sep.
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.
Returns a textual left-to-right alignment of boxes, where any two boxes are separated by sep.
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.
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’.
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’.
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.
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’.
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’.
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’.
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’.
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’.
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’.
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’.
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’.
Returns the maximum of its arguments; that is, the one box b in its arguments for which b > b1, b > b2, ... holds.
Returns the maximum of its arguments; that is, the one box b in its arguments for which b < b1, b < b2, ... holds.
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.
Returns a within a black rectangular frame of thickness thickness. thickness defaults to
rulethickness()(typically 1 pixel).
Returns a within a white rectangular frame of thickness thickness. thickness defaults to
whitethickness()(typically 2 pixels).
Returns a within a rectangular frame. Equivalent to ‘ruleframe(whiteframe(a)’.
Returns box a centered horizontally within a (vertical) alignment.
Example: In ‘a | hcenter(b) | c’, b is centered relatively to a and c.
Returns box a centered vertically within a (horizontal) alignment.
Example: In ‘a & vcenter(b) & c’, b is centered relatively to a and c.
Returns box a centered vertically and horizontally within an alignment.
Example: In ‘100 ^ center(b)’, b is centered within a square of size 100.
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.
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.
Returns a in “poor man's bold”: it is drawn two times, displaced horizontally by one pixel.
Return a box where white space of width
indentamount()is placed left of box.
Indent amount to be used in
indent(); defaults to ‘" "’ (two spaces).
To retrieve the string from a composite box, use string():
To convert numbers to strings, use num():
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")
Shortcut for ‘num(a, 10)’, ‘num(a, 8)’, ‘num(a, 2)’, ‘num(a, 16)’, respectively.
The functions in this section require inclusion of the library list.vsl.
For themes, list.vsl need not be included explicitly.
Return the concatenation of the given lists. Typically written in inline form:
[1] :: [2] :: [3] ⇒ [1, 2, 3].
Returns list with elem appended at the end:
append([1, 2, 3], 4) ⇒ [1, 2, 3, 4]
Returns True (1) if x is an element of list; False (0) if not:
member(1, [1, 2, 3]) ⇒ true
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,
Returns the first element of list:
car([1, 2, 3]) ⇒ 1
Returns list without its first element:
cdr([1, 2, 3]) ⇒ [2, 3]
Returns the n-th element (starting with 0) of list:
elem([4, 5, 6], 0) ⇒ 4
Returns the position of elem in list (starting with 0):
pos(4, [1, 2, 4]) ⇒ 2
Returns list, with all elements elem removed:
delete([4, 5, 5, 6], 5) ⇒ [4, 6]
Returns list, with the first element elem removed:
select([4, 5, 5, 6], 5) ⇒ [4, 5, 6]
Returns sortened list (according to box size):
sort([7, 4, 9]) ⇒ [4, 7, 9]
Returns a list of all characters in the box s:
chars("abc") ⇒ ["a", "b", "c"]
Returns a string, pretty-printing the list:
list([4, 5, 6]) ⇒ "[4, 5, 6]"
The functions in this section require inclusion of the library tab.vsl.
For themes, tab.vsl need not be included explicitly.
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
Like
tab, but place delimiters (horizontal and vertical rules) around table elements.
Returns padded table element x. Its default definition is:
tab_elem([]) = tab_elem(0); // empty table tab_elem(x) = whiteframe(x); // padding
The functions in this section require inclusion of the library fonts.vsl.
For themes, fonts.vsl need not be included explicitly.
Returns box, with all strings set in font (a valid X11 font description)
Font weight specifier in
fontname()(see below).
Font slant Specifier in
fontname()(see below).
Font family specifier in
fontname()(see below).
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).
Default font family:
family_times().DDD replaces this as set in the DDD font preferences. Use ‘ddd --fonts’ to see the actual definitions.
Default font size:
(stdfontpixels(), stdfontpoints()).DDD replaces this as set in the DDD font preferences. Use ‘ddd --fonts’ to see the actual definitions.
Default font size (in pixels): 0, meaning to use
stdfontpoints()instead.
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 tostdfontsize()(see above).
The functions in this section require inclusion of the library colors.vsl.
For themes, colors.vsl need not be included explicitly.
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.
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.
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).
Returns an ellipse containing box. Example:
ellipse("START"). If box is omitted, the ellipse is stretchable and expands to the available space.
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.
Create a stretchable box with a line from the lower left to the upper right corner. thickness defaults to
slopethickness()(see below).
Create a stretchable box with a line from the upper left to the lower right corner. thickness defaults to
slopethickness()(see below).
Returns a box with an arrow pointing to the upper, left, lower, or right side, respectively.
Returns a box with an arrow pointing to the upper left, upper right, lower left, or lower right side, respectively.
This appendix describes the VSL language.
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
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).
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.
Any constant integer n evaluates to a number—that is, a non-stretchable empty square box with size (n, n).
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.
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.
if a then b else c fi
returns b if a is true, and c otherwise. Only one of b or c is evaluated.
elsif a2 then b2 else c fi
is equivalent to
else if a2 then b2 else c fi fi
a and b
is equivalent to
if a then b else 0 fi
a or b
is equivalent to
if a then 1 else b fi
not a
is equivalent to
if a then 0 else 1 fi
Actually, ‘not’ is realized as a function; See Negation Functions, for details.
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
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.
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.
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;
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
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.
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.
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;
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.
You can redefine a VSL function even after its original definition. You can
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.
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().
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.
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:
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
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(: Creating Lists(: Negation Functions(: Comparison Functions(: Arithmetic Functions(: Overlays(: Textual Composition(: Vertical Composition(: Horizontal Compositionand: VSL Boolean Operatorsannotation: Displaying Data Displaysappend: Creating Listsarc: Arc Basicsarcs.vsl: Arc Functionsarcthickness: Arc Basicsarray_color: Displaying Colorsbf: Font Selectionbi: Font Selectionbin: String Functionsbox: Box Dimensionsbuiltin.vsl: VSL Operatorscar: Accessing List Elementscdr: Accessing List Elementscenter: Centering Functionschanged_color: Displaying Colorschanged_value: Displaying Extra Propertieschars: Lists and Stringscircle: Custom Arc Functionscollapsed_array: Displaying Arrayscollapsed_list_value: Displaying Listscollapsed_pointer_value: Displaying Pointerscollapsed_reference_value: Displaying Referencescollapsed_sequence_value: Displaying Sequencescollapsed_simple_value: Displaying Simple Valuescollapsed_struct_value: Displaying Structscollapsed_text_value: Displaying Multi-Line Textscolor: Color Functionscolors.vsl: Color Functionscommalist: Textual Compositioncrossline: Emphasis Functionsddd.vsl: DDD VSL Functionsdec: String Functionsdelete: Manipulating Listsdereferenced_pointer_value: Displaying Pointersdisabled: Displaying Data Displaysdisabled_color: Displaying Colorsdisplay_box: Displaying Data Displaysdisplay_color: Displaying Colorsdoubleframe: Frame Functionsdoublestrike: Emphasis Functionsdtab: Table Functionse_arrow: Arrow Functionse_flush: Flushing Functionselem: Accessing List Elementsellipse: Custom Arc Functionselse: VSL Conditionalselsif: VSL Conditionalsempty_array: Displaying Arraysempty_list_value: Displaying Listsempty_struct_value: Displaying Structsfall: Slope Basicsfamily_courier: Font Name Selectionfamily_helvetica: Font Name Selectionfamily_new_century: Font Name Selectionfamily_times: Font Name Selectionfamily_typewriter: Font Name Selectionfi: VSL Conditionalsfill: Empty Spacefix: Controlling Stretchflat: Manipulating Listsfont: Font Basicsfontname: Font Name Selectionfonts.vsl: Font Functionsframe: Frame Functionshalign: Horizontal Compositionhcenter: Centering Functionshead: Accessing List Elementshex: String Functionshfill: Empty Spacehfix: Controlling Stretchhorizontal_array: Displaying Arrayshorizontal_unnamed_list: Displaying Listshorizontal_unnamed_struct: Displaying Structshralign: Horizontal Compositionhrule: Black Lineshspace: Box Dimensionshwhite: White Spaceif: VSL Conditionalsindent: Indentation Functionsindentamount: Indentation Functionsisatom: List Propertiesislist: List Propertiesit: Font Selectionlast: Accessing List Elementslength: List Propertieslet: VSL Local Variableslist: Lists and Stringslist.vsl: List Functionslist_color: Displaying Colorslist_member: Displaying Listslist_member_name: Displaying Listslist_value: Displaying Listsmax: Maximum and Minimum Functionsmember: List Propertiesmin: Maximum and Minimum Functionsn_arrow: Arrow Functionsn_flush: Flushing Functionsne_arrow: Arrow Functionsne_flush: Flushing Functionsnone: Displaying Data Displaysnot: VSL Boolean Operatorsnum: String Functionsnumeric_value: Displaying Simple Valuesnw_arrow: Arrow Functionsnw_flush: Flushing Functionsoct: String Functionsoctogon: Custom Slope Functionsor: VSL Boolean Operatorsoval: Custom Arc Functionsoverline: Emphasis Functionspointer_color: Displaying Colorspointer_value: Displaying Pointerspos: Accessing List Elementsprefix: List Propertiespunchcard: Custom Slope Functionsreference_color: Displaying Colorsreference_value: Displaying Referencesrepeated_value: Displaying Extra Propertiesreverse: Manipulating Listsrhomb: Custom Slope Functionsrise: Slope Basicsrm: Font Selectionrule: Black Linesruleframe: Frame Functionsrulethickness: Black Liness_arrow: Arrow Functionss_flush: Flushing Functionsse_arrow: Arrow Functionsse_flush: Flushing Functionsselect: Manipulating Listssemicolonlist: Textual Compositionsequence_value: Displaying Sequencesshadow: Displaying Shadowsshadow_color: Displaying Colorssimple_color: Displaying Colorssimple_value: Displaying Simple Valuesslant_italic: Font Name Selectionslant_unslanted: Font Name Selectionslopes.vsl: Slope Functionsslopethickness: Slope Basicssmall_bf: Displaying Fontssmall_bi: Displaying Fontssmall_it: Displaying Fontssmall_rm: Displaying Fontssmall_size: Displaying Fontssort: Manipulating Listssquare: Box Dimensionsstd.vsl: VSL Librarystdfontfamily: Font Defaultsstdfontpixels: Font Defaultsstdfontpoints: Font Defaultsstdfontsize: Font Defaultsstdfontslant: Font Defaultsstdfontweight: Font Defaultsstring: String Functionsstruct_color: Displaying Colorsstruct_member: Displaying Structsstruct_member_name: Displaying Structsstruct_value: Displaying Structssublist: List Propertiessuffix: List Propertiessw_arrow: Arrow Functionssw_flush: Flushing Functionstab: Table Functionstab.vsl: Table Functionstab_elem: Table Functionstail: Accessing List Elementstalign: Textual Compositiontext_color: Displaying Colorstext_value: Displaying Multi-Line Textsthen: VSL Conditionalsthickframe: Frame Functionstiny_bf: Displaying Fontstiny_bi: Displaying Fontstiny_it: Displaying Fontstiny_rm: Displaying Fontstiny_size: Displaying Fontstitle: Displaying Data Displaystitle_bf: Displaying Fontstitle_bi: Displaying Fontstitle_color: Displaying Colorstitle_it: Displaying Fontstitle_rm: Displaying Fontstlist: Textual Compositiontralign: Textual Compositiontwodim_array: Displaying Arraystwodim_array_elem: Displaying Arraysunderline: Emphasis Functionsvalign: Vertical Compositionvalue_bf: Displaying Fontsvalue_bi: Displaying Fontsvalue_box: Displaying Data Displaysvalue_it: Displaying Fontsvalue_rm: Displaying Fontsvcenter: Centering Functionsvertical_array: Displaying Arraysvertical_unnamed_list: Displaying Listsvertical_unnamed_struct: Displaying Structsvfill: Empty Spacevfix: Controlling Stretchvlist: Vertical Compositionvralign: Vertical Compositionvrule: Black Linesvspace: Box Dimensionsvwhite: White Spacew_arrow: Arrow Functionsw_flush: Flushing Functionsweight_bold: Font Name Selectionweight_medium: Font Name Selectionwhere: VSL Local Variableswhiteframe: Frame Functionswhitethickness: White Space[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.