1. FAQ
    1. General
      1. Why yet another Scheme implementation?
      2. Why call it 'Chicken'?
      3. What should I do if I find a bug?
      4. Why are values defined with define-foreign-variable or define-constant or define-inline not seen outside of the containing source file?
      5. How does cond-expand know which features are registered in used units?
      6. Why are constants defined by define-constant not honoured in case constructs?
      7. How can I enable case sensitive reading/writing in user code?
      8. How can I change match-error-control during compilation?
      9. Why doesn't CHICKEN support the full numeric tower by default?
      10. How can I specialize a generic function method to match instances of every class?
      11. Does CHICKEN support native threads?
      12. Does CHICKEN support Unicode strings?
      13. Why do I get an "Error: invalid syntax: ..." using 'match' and 'syntax-case'?
    2. Platform specific
      1. How do I generate a DLL under MS Windows (tm) ?
      2. How do I generate a GUI application under Windows(tm)?
      3. Compiling very large files under Windows with the Microsoft C compiler fails with a message indicating insufficient heap space.
      4. When I run csi inside an emacs buffer under Windows, nothing happens.
      5. I load compiled code dynamically in a Windows GUI application and it crashes.
      6. On Windows, csc.exe seems to be doing something wrong.
      7. On Windows source and/or output filenames with embedded whitespace are not found.
    3. Customization
      1. How do I run custom startup code before the runtime-system is invoked?
      2. How can I add compiled user passes?
    4. Compiled macros
      1. Why is define-macro complaining about unbound variables?
      2. Why isn't load properly loading my library of macros?
      3. Why is include unable to load my hygienic macros?
      4. Why are macros not visible outside of the compilation unit in which they are defined?
    5. Warnings and errors
      1. Why does my program crash when I use callback functions (from Scheme to C and back to Scheme again)?
      2. Why does the linker complain about a missing function _C_..._toplevel?
      3. Why does the linker complain about a missing function _C_toplevel?
      4. Why does my program crash when I compile a file with -unsafe or unsafe declarations?
      5. Why do I get a warning when I define a global variable named match?
      6. Why don't toplevel-continuations captured in interpreted code work?
      7. Why does define-reader-ctor not work in my compiled program?
      8. Why do built-in units, such as srfi-1, srfi-18, and posix fail to load?
      9. How can I increase the size of the trace shown when runtime errors are detected?
    6. Optimizations
      1. How can I obtain smaller executables?
      2. How can I obtain faster executables?
      3. Which non-standard procedures are treated specially when the extended-bindings or usual-integrations declaration or compiler option is used?
      4. Can I load compiled code at runtime?
    7. Garbage collection
      1. Why does a loop that doesn't cons still trigger garbage collections?
      2. Why do finalizers not seem to work in simple cases in the interpeter?
    8. Interpreter
      1. Does CSI support history and autocompletion?
      2. Does code loaded with load run compiled or interpreted?
    9. Extensions
      1. How can I install Chicken eggs to a non-default location?
      2. Can I install chicken eggs as a non-root user?

FAQ

This is the list of Frequently Asked Questions about Chicken Scheme. If you have a question not answered here, feel free to post to the chicken-users mailing list; if you consider your question general enough, feel free to add it to this list.

General

Why yet another Scheme implementation?

Since Scheme is a relatively simple language, a large number of implementations exist and each has its specific advantages and disadvantages. Some are fast, some provide a rich programming environment. Some are free, others are tailored to specific domains, and so on. The reasons for the existence of CHICKEN are:

Why call it 'Chicken'?

According to felix:

> Well, it's pretty boring, really: when I started the project and needed some > name, the first thing that met my eyes was the "chicken" (actually a disguised penguin) > from the Wallace + Gromit movie... > And then there is of course the ever occurring chicken-and-egg problem > with bootstrapped compilers.

What should I do if I find a bug?

Send e-mail to felix@call-with-current-continuation.org with some hints about the problem, like version/build of the compiler, platform, system configuration, code that causes the bug, etc.

Why are values defined with define-foreign-variable or define-constant or define-inline not seen outside of the containing source file?

Accesses to foreign variables are translated directly into C constructs that access the variable, so the Scheme name given to that variable does only exist during compile-time. The same goes for constant- and inline-definitions: The name is only there to tell the compiler that this reference is to be replaced with the actual value.

How does cond-expand know which features are registered in used units?

Each unit used via (declare (uses ...)) is registered as a feature and so a symbol with the unit-name can be tested by cond-expand during macro-expansion-time. Features registered using the register-feature! procedure are only available during run-time of the compiled file. You can use the eval-when form to register features at compile time.

Why are constants defined by define-constant not honoured in case constructs?

case expands into a cascaded if expression, where the first item in each arm is treated as a quoted list. So the case macro can not infer whether a symbol is to be treated as a constant-name (defined via define-constant) or a literal symbol.

How can I enable case sensitive reading/writing in user code?

To enable the read procedure to read symbols and identifiers case sensitive, you can set the parameter case-sensitivity to #t.

How can I change match-error-control during compilation?

Use eval-when, like this:

(eval-when (compile)
(match-error-control #:unspecified) )

Why doesn't CHICKEN support the full numeric tower by default?

The short answer:

% chicken-setup numbers
% csi -q
#;1> (use numbers)

The long answer:

There are a number of reasons for this:

- For most applications of Scheme fixnums (exact word-sized integers) and flonums (64-bit floating-point numbers) are more than sufficient;

- Interfacing to C is simpler;

- Dispatching of arithmetic operations is more efficient.

There is an extension based on the GNU Multiprecision Package that implements most of the full numeric tower, see numbers.

How can I specialize a generic function method to match instances of every class?

Specializing a method on <object> doesn't work on primitive data objects like numbers, strings, etc. so for example

(define-method (foo (x <my-class>)) ...)
(define-method (foo (x <object>)) ...)
(foo 123)

will signal an error, because to applicable method can be found. To specialize a method for primitive objects, use <top>:

(define-method (foo (x <top>)) ...)

Does CHICKEN support native threads?

Native threads are not supported for two reasons. One, the runtime system is not reentrant. Two, concurrency implemented properly would require mandatory locking of every object that could be potentially shared between two threads. The garbage-collection algorithm would then become much more complex and inefficient, since the location of every object has to be accessed via a thread synchronization protocol. Such a design would make native threads in Chicken essentially equivalent to Unix processes and shared memory.

For a different approach to concurrency, please see the mpi egg.

Does CHICKEN support Unicode strings?

Yes, as an extension.

By default all string and character functions operate bytewise, so that characters with an integer 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.

You 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:

(use iset syntax-case utf8)
(import utf8)

This 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.

Most 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 utf8 egg documentation for details.

Why do I get an "Error: invalid syntax: ..." using 'match' and 'syntax-case'?

The built-in 'match' macro is incompatible with 'syntax-case'. Use the matchable egg instead.

Platform specific

How do I generate a DLL under MS Windows (tm) ?

Use csc in combination with the -dll option:

C:\> csc foo.scm -dll

How do I generate a GUI application under Windows(tm)?

Invoke csc with the -windows option. Or pass the -DC_WINDOWS_GUI option to the C compiler and link with the GUI version of the runtime system (that's libchicken-gui[-static].lib. The GUI runtime displays error messages in a message box and does some rudimentary command-line parsing.

Compiling very large files under Windows with the Microsoft C compiler fails with a message indicating insufficient heap space.

It seems that the Microsoft C compiler can only handle files up to a certain size, and it doesn't utilize virtual memory as well as the GNU C compiler, for example. Try closing running applications. If that fails, try to break up the Scheme code into several library units.

When I run csi inside an emacs buffer under Windows, nothing happens.

Invoke csi with the -:c runtime option. Under Windows the interpreter thinks it is not running under control of a terminal and doesn't print the prompt and does not flush the output stream properly.

I load compiled code dynamically in a Windows GUI application and it crashes.

Code compiled into a DLL to be loaded dynamically must be linked with the same runtime system as the loading application. That means that all dynamically loaded entities (including extensions built and installed with chicken-setup) must be compiled with the -windows csc option.

On Windows, csc.exe seems to be doing something wrong.

The Windows development tools include a C# compiler with the same name. Either invoke csc.exe with a full pathname, or put the directory where you installed CHICKEN in front of the MS development tool path in the PATH environment variable.

On Windows source and/or output filenames with embedded whitespace are not found.

There is no current workaround. Do not use filenames with embedded whitespace for code. However, command names with embedded whitespace will work correctly.

Customization

How do I run custom startup code before the runtime-system is invoked?

When you invoke the C compiler for your translated Scheme source program, add the C compiler option -DC_EMBEDDED, or pass -embedded to the csc driver program, so no entry-point function will be generated (main()). When your are finished with your startup processing, invoke:

CHICKEN_main(argc, argv, C_toplevel);

where C_toplevel is the entry-point into the compiled Scheme code. You should add the following declarations at the head of your code:

#include "chicken.h"
extern void C_toplevel(C_word,C_word,C_word) C_noret;

How can I add compiled user passes?

To add a compiled user pass instead of an interpreted one, create a library unit and recompile the main unit of the compiler (in the file chicken.scm) with an additional uses declaration. Then link all compiler modules and your (compiled) extension to create a new version of the compiler, like this (assuming all sources are in the current directory):

% cat userpass.scm
;;;; userpass.scm - My very own compiler pass

(declare (unit userpass))

;; Perhaps more user passes/extensions are added:
(let ([old (user-pass)])
(user-pass
(lambda (x)
(let ([x2 (do-something-with x)])
	  (if old
	      (old x2)
	      x2) ) ) ) )
% csc -c -x userpass.scm
% csc chicken.scm -c -o chicken-extended.o -uses userpass
% gcc chicken-extended.o support.o easyffi.o compiler.o optimizer.o batch-driver.o c-platform.o \
c-backend.o userpass.o `csc -ldflags -libs` -o chicken-extended

On platforms that support it (Linux ELF, Solaris, Windows + VC++), compiled code can be loaded via -extend just like source files (see load in the User's Manual).

Compiled macros

Why is define-macro complaining about unbound variables?

Macro bodies that are defined and used in a compiled source-file are evaluated during compilation and so have no access to anything created with define. Use define-for-syntax instead.

Why isn't load properly loading my library of macros?

During 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.

Why is include unable to load my hygienic macros?

It is not sufficient for the included file to require the syntax-case extension. Call (require-extension syntax-case) before calling include.

Why are macros not visible outside of the compilation unit in which they are defined?

Macros are defined during compile time, so when a file has been compiled, the definitions are gone. An exception to this rule are macros defined with define-macro, which are also visible at run-time, i.e. in eval. To use macros defined in other files, use the include special form.

Warnings and errors

Why does my program crash when I use callback functions (from Scheme to C and back to Scheme again)?

There are two reasons why code involving callbacks can crash out of no apparent reason:

  1. 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.
  2. 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.

Why does the linker complain about a missing function _C_..._toplevel?

This message indicates that your program uses a library-unit, but that the object-file or library was not supplied to the linker. If you have the unit foo, which is contained in foo.o than you have to supply it to the linker like this (assuming a GCC environment):

% csc program.scm foo.o -o program

Why does the linker complain about a missing function _C_toplevel?

This means you have compiled a library unit as an application. When a unit-declaration (as in (declare (unit ...))) is given, then this file has a specially named toplevel entry procedure. Just remove the declaration, or compile this file to an object-module and link it to your application code.

Why does my program crash when I compile a file with -unsafe or unsafe declarations?

The compiler option -unsafe or the declaration (declare (unsafe)) disable certain safety-checks to improve performance, so code that would normally trigger an error will work unexpectedly or even crash the running application. It is advisable to develop and debug a program in safe mode (without unsafe declarations) and use this feature only if the application works properly.

Why do I get a warning when I define a global variable named match?

Even when the match unit is not used, the macros from that package are visible in the compiler. The reason for this is that macros can not be accessed from library units (only when explicitly evaluated in running code). To speed up macro-expansion time, the compiler and the interpreter both already provide the compiled match-... macro definitions. Macros shadowed lexically are no problem, but global definitions of variables named identically to (global) macros are useless - the macro definition shadows the global variable.

This problem can be solved using a different name or undefining the macro, like this:

(eval-when (compile eval) (undefine-macro! 'match))

Why don't toplevel-continuations captured in interpreted code work?

Consider the following piece of code:

  
(define k (call-with-current-continuation (lambda (k) k)))
(k k)

When compiled, this will loop endlessly. But when interpreted, (k k) will return to the read-eval-print loop! This happens because the continuation captured will eventually read the next toplevel expression from the standard-input (or an input-file if loading from a file). At the moment k was defined, the next expression was (k k). But when k is invoked, the next expression will be whatever follows after (k k). In other words, invoking a captured continuation will not rewind the file-position of the input source. A solution is to wrap the whole code into a (begin ...) expression, so all toplevel expressions will be loaded together.

Why does define-reader-ctor not work in my compiled program?

The following piece of code does not work as expected:

(eval-when (compile)
(define-reader-ctor 'integer->char integer->char) )
(print #,(integer->char 33))

The problem is that the compiler reads the complete source-file before doing any processing on it, so the sharp-comma form is encountered before the reader-ctor is defined. A possible solution is to include the file containing the sharp-comma form, like this:

(eval-when (compile)
(define-reader-ctor 'integer->char integer->char) )

(include "other-file")
;;; other-file.scm:
(print #,(integer->char 33))

Why do built-in units, such as srfi-1, srfi-18, and posix fail to load?

When you try to use a built-in unit such as srfi-18, you may get the following error:

#;1> (use srfi-18)
; loading library srfi-18 ...
Error: (load-library) unable to load library
srfi-18
"dlopen(libchicken.dylib, 9): image not found"                ;; on a Mac
"libchicken.so: cannot open shared object file: No such file or directory"  ;; Linux

Another symptom is that (require 'srfi-18) will silently fail.

This 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:

export DYLD_LIBRARY_PATH=~/scheme/chicken/lib:$DYLD_LIBRARY_PATH ;; Mac
export LD_LIBRARY_PATH=~/scheme/chicken/lib:$LD_LIBRARY_PATH    ;; Linux

How can I increase the size of the trace shown when runtime errors are detected?

When a runtime error is detected, Chicken will print the last entries from the trace of functions called (unless your executable was compiled with the -no-trace option. By default, only 16 entries will be shown. To increase this number pass the -:aN parameter to your executable.

Optimizations

How can I obtain smaller executables?

If you don't need eval or the stuff in the extras library unit, you can just use the library unit:

	(declare (uses library))
	(display "Hello, world!\n")

(Don't forget to compile with the -explicit-use option) Compiled with Visual C++ this generates an executable of around 240 kilobytes. It is theoretically possible to compile something without the library, but a program would have to implement quite a lot of support code on its own.

How can I obtain faster executables?

There are a number of declaration specifiers that should be used to speed up compiled files: declaring (standard-bindings) is mandatory, since this enables most optimizations. Even if some standard procedures should be redefined, you can list untouched bindings in the declaration. Declaring (extended-bindings) lets the compiler choose faster versions of certain internal library functions. This might give another speedup. You can also use the the usual-integrations declaration, which is identical to declaring standard-bindings and extended-bindings (note that usual-integrations is set by default). Declaring (block) tells the compiler that global procedures are not changed outside the current compilation unit, this gives the compiler some more opportunities for optimization. If no floating point arithmetic is required, then declaring (number-type fixnum) can give a big performance improvement, because the compiler can now inline most arithmetic operations. Declaring (unsafe) will switch off most safety checks. If threads are not used, you can declare (disable-interrupts). You should always use maximum optimizations settings for your C compiler. Good GCC compiler options on Pentium (and compatible) hardware are: -Os -fomit-frame-pointer -fno-strict-aliasing Some programs are very sensitive to the setting of the nursery (the first heap-generation). You should experiment with different nursery settings (either by compiling with the -nursery option or by using the -:s... runtime option).

Which non-standard procedures are treated specially when the extended-bindings or usual-integrations declaration or compiler option is used?

The following standard bindings are handled specially, depending on optimization options and compiler settings:

{{+}} {{*}} {{-}} {{/}} {{quotient}} {{eq?}} {{eqv?}} {{equal?}} {{apply}} {{c...r}} {{values}} {{call-with-values}}
{{list-ref}} {{null?}} {{length}} {{not}} {{char?}} {{string?}} {{symbol?}} {{vector?}} {{pair?}} {{procedure?}}
{{boolean?}} {{number?}} {{complex?}} {{rational?}} {{real?}} {{exact?}} {{inexact?}} {{list?}} {{eof-object?}}
{{string-ref}} {{string-set!}} {{vector-ref}} {{vector-set!}} {{char=?}} {{char<?}} {{char>?}} {{char<=?}} {{char>=?}}
{{char-numeric?}} {{char-alphabetic?}} {{char-whitespace?}} {{char-upper-case?}}
{{char-lower-case?}} {{char-upcae}} {{char-downcase}} {{list-tail}} {{assv}} {{memv}} {{memq}} {{assoc}}
{{member}} {{set-car!}} {{set-cdr!}} {{abs}} {{exp}} {{sin}} {{cos}} {{tan}} {{log}} {{asin}} {{acos}} {{atan}} {{sqrt}}
{{zero?}} {{positive?}} {{negative?}} {{vector-length}} {{string-length}} {{char->integer}}
{{integer->char}} {{inexact->exact}} {{=}} {{>}} {{<}} {{>=}} {{<=}} {{for-each}} {{map}} {{substring}}
{{string-append}} {{gcd}} {{lcm}} {{list}} {{exact->inexact}} {{string->number}} {{number->string}}
{{even?}} {{odd?}} {{remainder}} {{floor}} {{ceiling}} {{truncate}} {{round}} {{cons}} {{vector}} {{string}}
{{string=?}} {{string-ci=?}} {{make-vector}} {{call-with-current-continuation}}
{{write-char}} {{read-string}} 

The following extended bindings are handled specially:

bitwise-and bitwise-ior bitwise-xor bitwise-not bit-set? add1 sub1 fx+ fx- fx* fx/ fxmod fx= fx> fx>= fixnum? fxneg fxmax fxmin fxand fxior fxxor fxnot fxshl fxshr flonum? fp+ fp- fp* fp/ atom? fp= fp> fp>= fpneg fpmax fpmin arithmetic-shift signum flush-output thread-specific thread-specific-set! not-pair? null-list? print print* u8vector->blob/shared s8vector->blob/shared u16vector->blob/shared s16vector->blob/shared u32vector->blob/shared s32vector->blob/shared f32vector->blob/shared f64vector->blob/shared block-ref blob-size u8vector-length s8vector-length u16vector-length s16vector-length u32vector-length s32vector-length f32vector-length f64vector-length u8vector-ref s8vector-ref u16vector-ref s16vector-ref u32vector-ref s32vector-ref f32vector-ref f64vector-ref u8vector-set! s8vector-set! u16vector-set! s16vector-set! u32vector-set! s32vector-set! hash-table-ref block-set! number-of-slots first second third fourth null-pointer? pointer->object make-record-instance locative-ref locative-set! locative? locative->object identity cpu-time error call/cc any? substring=? substring-ci=? substring-index substring-index-ci

Can I load compiled code at runtime?

Yes. You can load compiled at code at runtime with load just as well as you can load Scheme source code. Compiled code will, of course, run faster.

To do this, pass to load a path for a shared object. Use a form such as (load "foo.so") and run csc -shared foo.scm to produce foo.so from foo.scm (at which point foo.scm will no longer be required).

Garbage collection

Why does a loop that doesn't cons still trigger garbage collections?

Under CHICKENs implementation policy, tail recursion is achieved simply by avoiding to return from a function call. Since the programs are CPS converted, a continuous sequence of nested procedure calls is performed. At some stage the stack-space has to run out and the current procedure and its parameters (including the current continuation) are stored somewhere in the runtime system. Now a minor garbage collection occurs and rescues all live data from the stack (the first heap generation) and moves it into the the second heap generation. Then the stack is cleared (using a longjmp) and execution can continue from the saved state. With this method arbitrary recursion (in tail- or non-tail position) can happen, provided the application doesn't run out of heap-space. (The difference between a tail- and a non-tail call is that the tail-call has no live data after it invokes its continuation - and so the amount of heap-space needed stays constant)

Why do finalizers not seem to work in simple cases in the interpeter?

Consider the following interaction in CSI:

#;1> (define x '(1 2 3))
#;2> (define (yammer x) (print x " is dead"))
#;3> (set-finalizer! x yammer)
(1 2 3)
#;4> (gc #t)
157812
#;5> (define x #f)
#;6> (gc #t)
157812
#;7>

While you might expect objects to be reclaimed and "(1 2 3) is dead" printed, it won't happen: the literal list gets held in the interpreter history, because it is the result value of the set-finalizer! call. Running this in a normal program will work fine.

When testing finalizers from the interpreter, you might want to define a trivial macro such as

(define-macro (v x) `(begin (print ,x) (void)))

and wrap calls to set-finalizer! in it.

Interpreter

Does CSI support history and autocompletion?

CSI doesn't support it natively but it can be activated with the http://www.call-with-current-continuation.org/eggs/readline.html egg. After installing the egg, add the following to your ~/.csirc or equivalent file:

(require-extension readline)
(current-input-port (make-gnu-readline-port))
(gnu-history-install-file-manager (string-append (or (getenv "HOME") ".") "/.csi.history"))

Users of *nix-like systems (including Cygwin), may also want to check out 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.

Does code loaded with load run compiled or interpreted?

If you compile a file with a call to load, the code will be loaded at runtime and, if the file loaded is a Scheme source code file (instead of a shared object), it will be interpreted (even if the caller program is compiled).

Extensions

How can I install Chicken eggs to a non-default location?

You can just set the CHICKEN_REPOSITORY environment variable. It should contain the path where you want eggs to be installed:

$ export CHICKEN_REPOSITORY=~/chicken/
$ chicken-setup extensionname

In order to make programs (including csi) see these eggs, you should set this variable when you run them. Alternatively, you can call the repository-path Scheme procedure before loading the eggs, as in:

(repository-path "/home/azul/chicken")
(use format-modular)

Note, 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.

Can I install chicken eggs as a non-root user?

Yes, just install them in a directory you can write.


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