Script-fu In GIMP 2.4
Since version 1.0 of GIMP, it has included a powerful scripting language which permits extending the program's capabilities and simplifying repetitive tasks. This scripting language, called "Script-fu", was based upon the Scheme programming language and implemented the SIOD interpreter written by George J. Carrette while he was a professor at Boston University in the late 80s.
This Script-fu interpreter based upon Carrette's SIOD has served GIMP extremely well over the last decade -- thousands of scripts have been written and shared by GIMP users -- but it is starting to show its age and therefore the GIMP development team has decided to replace it with a more modern Scheme interpreter called TinyScheme. One of the main reasons for this changeover is to support international languages and fonts, for which SIOD offered no provision. There are other benefits as well, but lack of international support was the most significant.
Though this switch has required an extensive effort on the part of GIMP developers (particularly Kevin Cozens) and some significant changes to the internals of the GIMP code, there should be very little visible change to GIMP users. GIMP's scripting extension is still called "Script-fu" and the vast majority of the scripts already available will still function using the new TinyScheme-based interpreter.
Despite the desire to keep the impact of this change to GIMP internals to a minimum, there are some differences between the SIOD-based interpreter and the TinyScheme-based Script-fu which may crop up when trying to use older scripts with GIMP 2.4 and more recent releases. What follows is a description of some of the problems which may be encountered and what steps need to be taken to correct them.
- Setting an undeclared variable (Error: set!: unbound variable: x)
- Using the empty list in conditionals
- Accessing the first element of an empty list (Error: car: argument 1 must be: pair)
- Accessing beyond the last element of a list (Error: cdr: argument 1 must be: pair)
- Constructing a pair (Error: cons: needs 2 argument(s))
- Fractional numbers must not start with a dot (Error: eval: unbound variable: . )
- Deprecated features
- Conclusion (and other differences)
Setting an undeclared variable
By far, the most common problem that can be expected if using an
older script is that it might assign a value to a variable without
first declaring the variable. SIOD-based Script-fu would permit a
statement such as
(set! x 4) even if 'x' had
not been declared -- 'x' would be defined automatically to
be a global variable. The new Script-fu protects against this
situation and the programmer must declare the
variable first. The offending script would result in an error message
stating, "Error: set!: unbound variable: x".
The use of global variables is generally discouraged because
another function (written by a different author) may have chosen to
use the same name and the two functions would interfere with each
other. For this reason, the correct method of declaring 'x'
in the preceding example is to use the
(let* ( (x 4) ) ... ... ... )
The empty list in conditionals
SIOD treated the empty list to be FALSE when it appeared in a
conditional test (if, while, cond, not, =, etc) whereas the Scheme
standard specifies that it should evaluate to TRUE. Programmers have
been aware of this difference since the beginning and it is unlikely
that scripts will be encountered which rely upon SIOD's nonstandard
behavior but it is possible. A simple solution is to use the 'pair?'
function to test the list. For example, replace
... ) with
(while (pair? lis)
(not (null? lis)) could be used
Accessing the first element of an empty list
In SIOD, taking the 'car' of an empty list returned an empty list; in TinyScheme this is not permissible and will generate an error message ("Error: car: argument 1 must be: pair"). Like the case for conditionals, programmers have been aware of SIOD's nonstandard behavior and encountering this problem should be rare. Correcting such a problem, if encountered, should consist of testing whether a list is empty before accessing it.
Accessing beyond the last element of a list
Similar to the preceding problem, SIOD would permit you to access beyond the last element in a list, returning an empty list as a result. For example, taking the 'cdr' of an empty list or the 'cddr' of a one-element list. In GIMP 2.4, Script-fu will not allow this and it will result in an error message ("Error: cdr: argument 1 must be: pair"). Again, SIOD's behavior has long been realized to be non-standard and this problem's occurance should be rare. Correcting such a problem, if encountered, should consist of more precise testing when accessing a list.
Constructing a pair
cons function expects two arguments which
are combined into a pair. In SIOD, if only one argument was provided
then the second argument was assumed to be an empty list. In GIMP 2.4,
if the second argument is not present than an error occurs
("Error: cons: needs 2 argument(s)"). The
solution, should this problem be encountered, is explicitly include an
empty list as the second argument.
Fractional numbers must not start with a dot
If you had some numbers written as '.5' instead of '0.5', then you may get the error "Error: eval: unbound variable: . ". The solution is to make sure that all numbers start with a digit and add a leading '0' if necessary. (Note: this is considered as a bug and this may be fixed in a future GIMP release.)
The following SIOD functions or constants are currently made available in TinyScheme Script-fu but may disappear in future versions.
aset- replaced by TinyScheme's
aref- replaced by TinyScheme's
fopen- replaced by TinyScheme's
mapcar- replaced by TinyScheme's
nil- replaced by TinyScheme's
nreverse- replaced by TinyScheme's
pow- replaced by TinyScheme's
prin1- replaced by TinyScheme's
strcat- replaced by TinyScheme's
string-lessp- replaced by TinyScheme's
symbol-bound?- replaced by TinyScheme's
the-environment- replaced by TinyScheme's
*pi*- the constant *pi* is not predefined in TinyScheme but can be defined as
(* 4 (atan 1.0))
butlast- is not available in TinyScheme but alternate coding using
(reverse (cdr (reverse x)))is possible
cons-array- replaced by TinyScheme's
There are some other differences between the original Script-fu and
the Script-fu of GIMP 2.4 but they should have little or no impact on
existing scripts because of their rarity. These include the syntax for
throw statements (which trap
errors) and the
bytes-append function (which does not
seem to appear in any published Script-fu). If you encounter scripts
containing such problems, please post on the GIMP developers mailing
list outlining your problems.
More information about the Scheme syntax of Script-Fu can be found in the Revised5 Report on the Algorithmic Language Scheme, also know as R5RS. Tinyscheme does not support all features of R5RS, but if a precedure is available, it is supposed to behave like documented.