source: project/chicken/trunk/manual/faq @ 13452

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1[[toc:]]
2[[tags:faq]]
3
4== FAQ
5
6This is the list of Frequently Asked Questions about Chicken Scheme.
7If you have a question not answered here, feel free to post to the chicken-users mailing list;
8if you consider your question general enough, feel free to add it to this list.
9
10=== General
11
12==== Why yet another Scheme implementation?
13
14Since Scheme is a relatively simple language, a large number of implementations exist and
15each has its specific advantages and disadvantages. Some are fast, some provide a rich
16programming environment. Some are free, others are tailored to specific domains, and so on. The reasons
17for the existence of CHICKEN are:
18
19* CHICKEN is portable because it generates C code that runs on a large number of platforms.
20
21* CHICKEN is extendable, since its code generation scheme and runtime system/garbage collector fits neatly into a C environment.
22
23* CHICKEN is free and can be freely distributed, including its source code.
24
25* CHICKEN offers better performance than nearly all interpreter based implementations, but still provides full Scheme semantics.
26
27* As far as we know, CHICKEN is the first implementation of Scheme that uses Henry Baker's [[http://home.pipeline.com/~hbaker1/CheneyMTA.html|Cheney on the M.T.A]] concept.
28
29==== Why call it 'Chicken'?
30
31According to [[http://chicken.wiki.br/felix winkelmann|felix]]:
32
33> Well, it's pretty boring, really: when I started the project and needed some
34> name, the first thing that met my eyes was the "chicken" (actually a disguised penguin)
35> from the Wallace + Gromit movie...
36> And then there is of course the ever occurring chicken-and-egg problem
37> with bootstrapped compilers.
38
39==== What should I do if I find a bug?
40
41Send e-mail to felix@call-with-current-continuation.org
42with some hints about the problem, like
43version/build of the compiler, platform, system configuration, code that
44causes the bug, etc.
45
46==== Why are values defined with {{define-foreign-variable}} or {{define-constant}} or {{define-inline}} not seen outside of the containing source file?
47
48Accesses to foreign variables are translated directly into C constructs that access the variable,
49so the Scheme name given to that variable does only exist during compile-time.
50The same goes for constant- and inline-definitions: The name is only there to tell the compiler
51that this reference is to be replaced with the actual value.
52
53==== How does {{cond-expand}} know which features are registered in used units?
54
55Each unit used via {{(declare (uses ...))}} is registered as a feature and
56so a symbol with the unit-name can be tested by {{cond-expand}} during macro-expansion-time.
57Features registered using the {{register-feature!}} procedure are only
58available during run-time of the compiled file. You can use the {{eval-when}} form
59to register features at compile time.
60
61==== Why are constants defined by {{define-constant}} not honoured in {{case}} constructs?
62
63{{case}} expands into a cascaded {{if}} expression, where the first item in each arm
64is treated as a quoted list. So the {{case}} macro can not infer whether
65a symbol is to be treated as a constant-name (defined via {{define-constant}}) or
66a literal symbol.
67
68
69==== How can I enable case sensitive reading/writing in user code?
70
71To enable the {{read}} procedure to read symbols and identifiers case sensitive, you can set the
72parameter {{case-sensitivity}} to {{#t}}.
73
74
75==== Why doesn't CHICKEN support the full numeric tower by default?
76
77The short answer:
78
79<enscript highlight=scheme>
80% chicken-install numbers
81% csi -q
82#;1> (use numbers)
83</enscript>
84
85The long answer:
86
87There are a number of reasons for this:
88
89- For most applications of Scheme fixnums (exact word-sized integers) and flonums (64-bit floating-point
90numbers) are more than sufficient;
91
92- Interfacing to C is simpler;
93
94- Dispatching of arithmetic operations is more efficient.
95
96There is an extension based on the GNU Multiprecision Package that implements most of the full
97numeric tower, see [[http://chicken.wiki.br/numbers|numbers]].
98
99
100==== How can I specialize a generic function method to match instances of every class?
101
102Specializing a method on {{<object>}} doesn't work on primitive data objects like
103numbers, strings, etc. so for example
104
105<enscript highlight=scheme>
106(define-method (foo (x <my-class>)) ...)
107(define-method (foo (x <object>)) ...)
108(foo 123)
109</enscript>
110
111will signal an error, because to applicable method can be found. To specialize a method for primitive
112objects, use {{<top>}}:
113
114<enscript highlight=scheme>
115(define-method (foo (x <top>)) ...)
116</enscript>
117
118
119==== Does CHICKEN support native threads?
120
121Native threads are not supported for two reasons. One, the runtime
122system is not reentrant.  Two, concurrency implemented properly would
123require mandatory locking of every object that could be potentially
124shared between two threads. The garbage-collection algorithm would
125then become much more complex and inefficient, since the location of
126every object has to be accessed via a thread synchronization
127protocol. Such a design would make native threads in Chicken
128essentially equivalent to Unix processes and shared memory.
129
130For a different approach to concurrency, please see the
131[[http://www.call-with-current-continuation.org/eggs/3/mpi.html|mpi]]
132egg.
133
134==== Does CHICKEN support Unicode strings?
135
136Yes, as an extension.
137
138By default all string and character functions operate bytewise, so that characters with an iteger value greater than 255 don't make much sense and multibyte UTF-8 characters are seen and manipulated as separate bytes, analogous to what a C program would see.
139
140You can enable UTF-8 support by placing the following two lines at the beginning of your source file (or in your ~/.csirc for interactive sessions) before any other code, including other use directives:
141
142<enscript highlight=scheme>
143(use iset syntax-case utf8)
144(import utf8)
145</enscript>
146
147This will replace all builtin string operators with UTF-8-aware versions, that will treat strings as sequences of multibyte UTF-8 characters, thus enabling you to represent and manipulate Unicode characters while remaining compatible with most C libraries and system interfaces.
148
149Most eggs should work correctly in utf8 mode, including the regex extension, but you still have the option of working around incompatibilities of specific eggs by loading them before the (import utf8) directive. Keep in mind that some operations, such as string-length, are much more expensive in utf8 (multibyte) mode, and should be used with care. See the [[http://www.call-with-current-continuation.org/eggs/utf8.html|utf8 egg documentation]] for details.
150
151
152==== Why do I get an "Error: invalid syntax: ..." using 'match' and 'syntax-case'?
153
154The built-in 'match' macro is incompatible with 'syntax-case'. Use the  [[http://www.call-with-current-continuation.org/eggs/matchable.html|matchable egg]] instead.
155
156
157=== Platform specific
158
159==== How do I generate a DLL under MS Windows (tm) ?
160
161Use {{csc}} in combination with the {{-dll}} option:
162
163{{C:\> csc foo.scm -dll}}
164
165==== How do I generate a GUI application under Windows(tm)?
166
167Invoke {{csc}} with the {{-windows}} option. Or pass the {{-DC_WINDOWS_GUI}}
168option to the C compiler and link with the GUI version of the runtime system (that's {{libchicken-gui[-static].lib}}.
169The GUI runtime displays error messages in a message box and does some rudimentary command-line
170parsing.
171
172==== Compiling very large files under Windows with the Microsoft C compiler fails with a message indicating insufficient heap space.
173
174It seems that the Microsoft C compiler can only handle files up to a certain size, and it doesn't utilize virtual memory as
175well as the GNU C compiler, for example. Try closing running applications. If that fails, try to break up the Scheme code
176into several library units.
177
178==== When I run {{csi}} inside an emacs buffer under Windows, nothing happens.
179
180Invoke {{csi}} with the {{-:c}} runtime option. Under Windows the interpreter thinks it
181is not running under control of a terminal and doesn't print the prompt and does not flush the output stream properly.
182
183==== I load compiled code dynamically in a Windows GUI application and it crashes.
184
185Code compiled into a DLL to be loaded dynamically must be linked with the same runtime system as the loading
186application. That means that all dynamically loaded entities (including extensions built and installed with
187{{chicken-install}}) must be compiled with the {{-windows}} {{csc}} option.
188
189==== On Windows, {{csc.exe}} seems to be doing something wrong.
190
191The Windows development tools include a C# compiler with the same name. Either invoke {{csc.exe}} with a full
192pathname, or put the directory where you installed CHICKEN in front of the MS development tool path in the {{PATH}}
193environment variable.
194
195==== On Windows source and/or output filenames with embedded whitespace are not found.
196
197There is no current workaround. Do not use filenames with embedded whitespace for code. However, command
198names with embedded whitespace will work correctly.
199=== Customization
200
201
202==== How do I run custom startup code before the runtime-system is invoked?
203
204When you invoke the C compiler for your translated Scheme source program, add the C compiler option
205{{-DC_EMBEDDED}}, or pass {{-embedded}} to the {{csc}}
206driver program, so no entry-point function will be generated ({{main()}}).
207When your are finished with your startup processing, invoke:
208
209<enscript highlight=c>
210CHICKEN_main(argc, argv, C_toplevel);
211</enscript>
212
213where {{C_toplevel}} is the entry-point into the compiled Scheme code. You
214should add the following  declarations at the head of your code:
215
216<enscript highlight=c>
217#include "chicken.h"
218extern void C_toplevel(C_word,C_word,C_word) C_noret;
219</enscript>
220
221==== How can I add compiled user passes?
222
223To add a compiled user pass instead of an interpreted one, create a library unit and recompile
224the main unit of the compiler (in the file {{chicken.scm}}) with an additional {{uses}}
225declaration. Then link all compiler modules and your (compiled) extension to create a new version of
226the compiler, like this (assuming all sources are in the
227current directory):
228
229<enscript highlight=scheme>
230% cat userpass.scm
231;;;; userpass.scm - My very own compiler pass
232
233(declare (unit userpass))
234
235;; Perhaps more user passes/extensions are added:
236(let ([old (user-pass)])
237(user-pass
238(lambda (x)
239(let ([x2 (do-something-with x)])
240          (if old
241              (old x2)
242              x2) ) ) ) )
243</enscript>
244
245 % csc -c -x userpass.scm
246 % csc chicken.scm -c -o chicken-extended.o -uses userpass
247 % gcc chicken-extended.o support.o easyffi.o compiler.o optimizer.o batch-driver.o c-platform.o \
248 c-backend.o userpass.o `csc -ldflags -libs` -o chicken-extended
249
250On platforms that support it (Linux ELF, Solaris, Windows + VC++), compiled code can be loaded via {{-extend}}
251just like source files (see {{load}} in the User's Manual).
252
253
254=== Compiled macros
255
256==== Why are low-level macros defined with {{define-syntax}} complaining about unbound variables?
257
258Macro bodies that are defined and used in a compiled source-file are
259evaluated during compilation and so have no access to anything created with {{define}}. Use {{define-for-syntax}} instead.
260
261==== Why isn't {{load}} properly loading my library of macros?
262
263During compile-time, macros are only available in the source file in which they are defined. Files included via {{include}} are considered part of the containing file.
264
265==== Why is {{include}} unable to load my hygienic macros?
266
267It is not sufficient for the included file to require the {{syntax-case}} extension. Call {{(require-extension syntax-case)}} ''before'' calling {{include}}.
268
269==== Why are macros not visible outside of the compilation unit in which they are defined?
270
271Macros are defined during compile time, so when a file has been compiled, the definitions are gone. An exception
272to this rule are macros defined with {{define-macro}}, which are also visible at run-time, i.e.
273in {{eval}}. To use macros defined in other files, use the {{include}} special
274form.
275
276=== Warnings and errors
277
278==== Why does my program crash when I use callback functions (from Scheme to C and back to Scheme again)?
279
280There are two reasons why code involving callbacks can crash out of no apparent reason:
281
282# It is important to use {{foreign-safe-lambda/foreign-safe-lambda*}} for the C code that is to call back into Scheme. If this is not done than sooner or later the available stack space will be exhausted.
283
284# If the C code uses a large amount of stack storage, or if Scheme-to-C-to-Scheme calls are nested deeply, then the available nursery space on the stack will run low. To avoid this it might be advisable to run the compiled code with a larger nursery setting, i.e. run the code with {{-:s...}} and a larger value than the default (for example {{-:s300k}}), or use the {{-nursery}} compiler option.  Note that this can decrease runtime performance on some platforms.
285
286==== Why does the linker complain about a missing function {{_C_..._toplevel}}?
287
288This message indicates that your program uses a library-unit, but that the
289object-file or library was not supplied to the linker. If you have the unit
290{{foo}}, which is contained in {{foo.o}} than you have to supply it to the
291linker like this (assuming a GCC environment):
292
293{{% csc program.scm foo.o -o program}}
294
295==== Why does the linker complain about a missing function {{_C_toplevel}}?
296
297This means you have compiled a library unit as an application. When a unit-declaration (as in {{(declare (unit ...))}})
298is given, then this file has a specially named toplevel entry procedure. Just remove the declaration,
299or compile this file to an object-module and link it to your application code.
300
301==== Why does my program crash when I compile a file with {{-unsafe}} or unsafe declarations?
302
303The compiler option {{-unsafe}} or the declaration {{(declare (unsafe))}} disable
304certain safety-checks to improve performance, so code that would normally
305trigger an error will work unexpectedly or even crash the running application.
306It is advisable to develop and debug a program in safe mode (without unsafe
307declarations) and use this feature only if the application works properly.
308
309==== Why do I get a warning when I define a global variable named {{match}}?
310
311Even when the {{match}} unit is not used, the macros from that package are visible in the compiler.
312The reason for this is that macros can not be accessed from library units (only when explicitly evaluated in running
313code). To speed up macro-expansion time, the compiler and the interpreter both already provide the compiled
314{{match-...}} macro definitions. Macros shadowed lexically are no problem, but global definitions
315of variables named identically to (global) macros are useless - the macro definition shadows the global
316variable.
317
318This problem can be solved using a different name or undefining the macro, like this:
319
320<enscript highlight=scheme>
321(eval-when (compile eval) (undefine-macro! 'match))
322</enscript>
323==== Why don't toplevel-continuations captured in interpreted code work?
324
325Consider the following piece of code:
326
327<enscript highlight=scheme> 
328(define k (call-with-current-continuation (lambda (k) k)))
329(k k)
330</enscript>
331
332When compiled, this will loop endlessly. But when interpreted, {{(k k)}} will return
333to the read-eval-print loop! This happens because the continuation captured will eventually read the
334next toplevel expression from the standard-input (or an input-file if loading from a file). At the moment
335{{k}} was defined, the next expression was {{(k k)}}. But when {{k}}
336is invoked, the next expression will be whatever follows after {{(k k)}}.
337In other words, invoking a captured continuation will not rewind the file-position of the input source.
338A solution is to wrap the whole code into a {{(begin ...)}} expression, so all toplevel
339expressions will be loaded together.
340
341==== Why does {{define-reader-ctor}} not work in my compiled program?
342
343The following piece of code does not work as expected:
344
345<enscript highlight=scheme>
346(eval-when (compile)
347(define-reader-ctor 'integer->char integer->char) )
348(print #,(integer->char 33))
349</enscript>
350
351The problem is that the compiler reads the complete source-file before doing any processing on it,
352so the sharp-comma form is encountered before the reader-ctor is defined. A possible solution is to include
353the file containing the sharp-comma form, like this:
354
355<enscript highlight=scheme>
356(eval-when (compile)
357(define-reader-ctor 'integer->char integer->char) )
358
359(include "other-file")
360</enscript>
361
362<enscript highlight=scheme>
363;;; other-file.scm:
364(print #,(integer->char 33))
365</enscript>
366
367==== Why do built-in units, such as srfi-1, srfi-18, and posix fail to load?
368
369When you try to {{use}} a built-in unit such as {{srfi-18}}, you may get the following error:
370
371<enscript highlight=scheme>
372#;1> (use srfi-18)
373; loading library srfi-18 ...
374Error: (load-library) unable to load library
375srfi-18
376"dlopen(libchicken.dylib, 9): image not found"                ;; on a Mac
377"libchicken.so: cannot open shared object file: No such file or directory"  ;; Linux
378</enscript>
379
380Another symptom is that {{(require 'srfi-18)}} will silently fail.
381
382This typically happens because the Chicken libraries have been installed in a non-standard location, such as your home directory.  The workaround is to explicitly tell the dynamic linker where to look for your libraries:
383
384 export DYLD_LIBRARY_PATH=~/scheme/chicken/lib:$DYLD_LIBRARY_PATH ;; Mac
385 export LD_LIBRARY_PATH=~/scheme/chicken/lib:$LD_LIBRARY_PATH    ;; Linux
386
387==== How can I increase the size of the trace shown when runtime errors are detected?
388
389When a runtime error is detected, Chicken will print the last entries from the trace of functions called
390(unless your executable was compiled with the {{-no-trace}} option.
391By default, only 16 entries will be shown.
392To increase this number pass the {{-:aN}} parameter to your executable.
393
394=== Optimizations
395
396==== How can I obtain smaller executables?
397
398If you don't need {{eval}} or the stuff in the {{extras}} library unit,
399you can just use the {{library}} unit:
400
401<enscript highlight=scheme>
402        (declare (uses library))
403        (display "Hello, world!\n")
404</enscript>
405
406(Don't forget to compile with the {{-explicit-use}} option)
407Compiled with Visual C++ this generates an executable of around 240 kilobytes.
408It is theoretically possible to compile something without the library, but
409a program would have to implement quite a lot of support code on its own.
410
411==== How can I obtain faster executables?
412
413There are a number of declaration specifiers that should be used to speed up
414compiled files: declaring {{(standard-bindings)}} is mandatory, since this enables
415most optimizations. Even if some standard procedures should be redefined, you can
416list untouched bindings in the declaration.
417Declaring {{(extended-bindings)}} lets the compiler choose faster versions of certain
418internal library functions. This might give another speedup. You can also use the
419the {{usual-integrations}} declaration, which is identical to declaring
420{{standard-bindings}} and {{extended-bindings}}
421(note that {{usual-integrations}} is set by default).
422Declaring {{(block)}} tells the compiler that global procedures are not changed
423outside the current compilation unit, this gives the compiler some more
424opportunities for optimization.
425If no floating point arithmetic is required, then declaring {{(number-type fixnum)}}
426can give a big performance improvement, because the compiler can now inline
427most arithmetic operations.
428Declaring {{(unsafe)}} will switch off most safety checks.
429If threads are not used, you can declare {{(disable-interrupts)}}.
430You should always use maximum optimizations settings for your C compiler.
431Good GCC compiler options on Pentium (and compatible) hardware are:
432{{-Os -fomit-frame-pointer -fno-strict-aliasing}}
433Some programs are very sensitive to the setting of the nursery (the first heap-generation). You
434should experiment with different nursery settings (either by compiling with the {{-nursery}}
435option or by using the {{-:s...}} runtime option).
436
437==== Which non-standard procedures are treated specially when the {{extended-bindings}} or {{usual-integrations}} declaration or compiler option is used?
438
439The following standard bindings are handled specially, depending on optimization options
440and compiler settings:
441
442 {{+}} {{*}} {{-}} {{/}} {{quotient}} {{eq?}} {{eqv?}} {{equal?}} {{apply}} {{c...r}} {{values}} {{call-with-values}}
443 {{list-ref}} {{null?}} {{length}} {{not}} {{char?}} {{string?}} {{symbol?}} {{vector?}} {{pair?}} {{procedure?}}
444 {{boolean?}} {{number?}} {{complex?}} {{rational?}} {{real?}} {{exact?}} {{inexact?}} {{list?}} {{eof-object?}}
445 {{string-ref}} {{string-set!}} {{vector-ref}} {{vector-set!}} {{char=?}} {{char<?}} {{char>?}} {{char<=?}} {{char>=?}}
446 {{char-numeric?}} {{char-alphabetic?}} {{char-whitespace?}} {{char-upper-case?}}
447 {{char-lower-case?}} {{char-upcae}} {{char-downcase}} {{list-tail}} {{assv}} {{memv}} {{memq}} {{assoc}}
448 {{member}} {{set-car!}} {{set-cdr!}} {{abs}} {{exp}} {{sin}} {{cos}} {{tan}} {{log}} {{asin}} {{acos}} {{atan}} {{sqrt}}
449 {{zero?}} {{positive?}} {{negative?}} {{vector-length}} {{string-length}} {{char->integer}}
450 {{integer->char}} {{inexact->exact}} {{=}} {{>}} {{<}} {{>=}} {{<=}} {{for-each}} {{map}} {{substring}}
451 {{string-append}} {{gcd}} {{lcm}} {{list}} {{exact->inexact}} {{string->number}} {{number->string}}
452 {{even?}} {{odd?}} {{remainder}} {{floor}} {{ceiling}} {{truncate}} {{round}} {{cons}} {{vector}} {{string}}
453 {{string=?}} {{string-ci=?}} {{make-vector}} {{call-with-current-continuation}}
454 {{write-char}} {{read-string}}
455
456The following extended bindings are handled specially:
457
458{{bitwise-and}} {{bitwise-ior}} {{bitwise-xor}} {{bitwise-not}}
459{{bit-set?}} {{add1}} {{sub1}}
460{{fx+}}
461{{fx-}} {{fx*}} {{fx/}} {{fxmod}}
462{{fx=}} {{fx>}} {{fx>=}} {{fixnum?}} {{fxneg}} {{fxmax}} {{fxmin}}
463{{fxand}} {{fxior}} {{fxxor}} {{fxnot}} {{fxshl}} {{fxshr}}
464{{flonum?}} {{fp+}}
465{{fp-}} {{fp*}} {{fp/}} {{atom?}}
466{{fp=}} {{fp>}} {{fp>=}} {{fpneg}} {{fpmax}} {{fpmin}}
467{{arithmetic-shift}} {{signum}} {{flush-output}} {{thread-specific}} {{thread-specific-set!}}
468{{not-pair?}} {{null-list?}} {{print}} {{print*}} {{u8vector->blob/shared}}
469{{s8vector->blob/shared}} {{u16vector->blob/shared}} {{s16vector->blob/shared}}
470{{u32vector->blob/shared}}
471{{s32vector->blob/shared}} {{f32vector->blob/shared}} {{f64vector->blob/shared}} {{block-ref}}
472{{blob-size}}
473{{u8vector-length}}
474{{s8vector-length}}
475{{u16vector-length}}
476{{s16vector-length}}
477{{u32vector-length}}
478{{s32vector-length}}
479{{f32vector-length}}
480{{f64vector-length}}
481{{u8vector-ref}}
482{{s8vector-ref}}
483{{u16vector-ref}}
484{{s16vector-ref}}
485{{u32vector-ref}}
486{{s32vector-ref}}
487{{f32vector-ref}}
488{{f64vector-ref}}
489{{u8vector-set!}}
490{{s8vector-set!}}
491{{u16vector-set!}}
492{{s16vector-set!}}
493{{u32vector-set!}}
494{{s32vector-set!}}
495{{hash-table-ref}}
496{{block-set!}} {{number-of-slots}}
497{{first}} {{second}} {{third}} {{fourth}} {{null-pointer?}} {{pointer->object}}
498{{make-record-instance}}
499{{locative-ref}} {{locative-set!}} {{locative?}} {{locative->object}} {{identity}}
500{{cpu-time}} {{error}} {{call/cc}} {{any?}}
501{{substring=?}} {{substring-ci=?}} {{substring-index}} {{substring-index-ci}}
502
503==== Can I load compiled code at runtime?
504
505Yes.
506You can load compiled at code at runtime with {{load}} just as
507well as you can load Scheme source code.
508Compiled code will, of course, run faster.
509
510To do this, pass to {{load}} a path for a shared object.
511Use a form such as {{(load "foo.so")}} and run
512{{csc -shared foo.scm}} to produce {{foo.so}} from {{foo.scm}} (at
513which point {{foo.scm}} will no longer be required).
514
515=== Garbage collection
516
517==== Why does a loop that doesn't {{cons}} still trigger garbage collections?
518
519Under CHICKENs implementation policy, tail recursion is achieved simply by avoiding
520to return from a function call. Since the programs are CPS converted, a continuous
521sequence of nested procedure calls is performed. At some stage the stack-space has
522to run out and the current procedure and its parameters (including the current
523continuation) are stored somewhere in the runtime system. Now a minor garbage collection
524occurs and rescues all live
525data from the stack (the first heap generation) and moves it into the the second heap
526generation. Then the stack is cleared (using
527a {{longjmp}}) and execution can continue from the saved state.
528With this method arbitrary recursion (in tail- or non-tail position) can happen,
529provided the application doesn't run out of heap-space.
530(The difference between a tail- and a non-tail call is that the tail-call has no
531live data after it invokes its continuation - and so the amount of heap-space needed stays constant)
532
533==== Why do finalizers not seem to work in simple cases in the interpeter?
534
535Consider the following interaction in CSI:
536
537 #;1> (define x '(1 2 3))
538 #;2> (define (yammer x) (print x " is dead"))
539 #;3> (set-finalizer! x yammer)
540 (1 2 3)
541 #;4> (gc #t)
542 157812
543 #;5> (define x #f)
544 #;6> (gc #t)
545 157812
546 #;7>
547
548While you might expect objects to be reclaimed and "''(1 2 3) is dead''" printed, it won't happen:
549the literal list gets held in the interpreter history, because it is the
550result value of the set-finalizer! call.
551Running this in a normal program will work fine.
552
553When testing finalizers from the interpreter, you might want to define a trivial macro such as
554
555 (define-macro (v x) `(begin (print ,x) (void)))
556
557and wrap calls to {{set-finalizer!}} in it.
558
559=== Interpreter
560
561==== Does CSI support history and autocompletion?
562
563CSI doesn't support it natively but it can be activated with the [[http://www.call-with-current-continuation.org/eggs/readline.html]] egg.
564After installing the egg, add the following to your {{~/.csirc}} or equivalent file:
565
566<enscript highlight=scheme>
567(require-extension readline)
568(current-input-port (make-gnu-readline-port))
569(gnu-history-install-file-manager (string-append (or (getenv "HOME") ".") "/.csi.history"))
570</enscript>
571
572Users of *nix-like systems (including Cygwin), may also want to check out [[http://utopia.knoware.nl/~hlub/rlwrap/|rlwrap]].  This program lets you "wrap" another process (e.g. {{rlwrap csi}}) with the readline library, giving you history, autocompletion, and the ability to set the keystroke set. Vi fans can get vi keystrokes by adding "set editing-mode vi" to their {{.inputrc}} file.
573
574==== Does code loaded with {{load}} run compiled or interpreted?
575
576If you compile a file with a call to {{load}}, the code will be loaded at
577runtime and, if the file loaded is a Scheme source code file
578(instead of a shared object), it will be
579interpreted (even if the caller program is compiled).
580
581==== How do I use extended (non-standard) syntax in evaluated code at run-time?
582
583Normally, only standard Scheme syntax is available to the evaluator. To
584use the extensions provided in the CHICKEN compiler and interpreter,
585add:
586
587<enscript highlight=scheme>
588(require-library chicken-syntax)
589</enscript>
590
591=== Extensions
592
593==== How can I install Chicken eggs to a non-default location?
594
595You can just set the {{CHICKEN_REPOSITORY}} environment variable.
596It should contain the path where you want eggs to be installed:
597
598 $ export CHICKEN_REPOSITORY=~/chicken/
599 $ chicken-install extensionname
600
601In order to make programs (including csi) see these eggs, you should set this variable when you run them.
602Alternatively, you can call the {{repository-path}} Scheme procedure before loading the eggs, as in:
603
604<enscript highlight=scheme>
605(repository-path "/home/azul/chicken")
606(use format-modular)
607</enscript>
608
609Note, however, that using {{repository-path}} as above hard-codes the location of your eggs in your source files.  While this might not be an issue in your case, it might be safe to keep this configuration outside of the source code (that is, specifying it as an environment variable) to make it easier to maintain.
610
611==== Can I install chicken eggs as a non-root user?
612
613Yes, just [[FAQ#Extensions#How can I install Chicken eggs to a non-default location?|install them in a directory you can write]].
614
615==== Why does downloading an extension via {{chicken-install}} fail on Windows Vista?
616
617Possibly the Windows Firewall is active, which prevents {{chicken-install}} from opening a TCP
618connection to the egg repository. Try disabling the firewall temporarily.
619
620----
621
622Previous: [[Bugs and limitations]]
623
624Next: [[Acknowledgements]]
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