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:

CHICKEN is portable because it generates C code that runs on a large number of platforms.
CHICKEN is extensible, since its code generation scheme and runtime system/garbage collector fits neatly into a C environment.
CHICKEN is free and can be freely distributed, including its source code.
CHICKEN offers better performance than nearly all interpreter based implementations, but still provides full Scheme semantics.
As far as we know, CHICKEN is the first implementation of Scheme that uses Henry Baker's Cheney on the M.T.A concept.

What should I do if I find a bug?

Fill a ticket at bugs.call-cc.org with some hints about the problem, like version/build of the compiler, platform, system configuration, code that causes the bug, etc.

Specific

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.

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 or concurrent-native-callbacks egg.

Does CHICKEN support Unicode strings?

The system does not directly support Unicode, but there is an extension for UTF-8 strings: utf8.

Why are `dynamic-wind' thunks not executed when a thread signals an error?

Here is what Marc Feeley, the author of SRFI-18 has to say about this subject:

   >No the default exception handler shouldn't invoke the after
   > thunks of the current continuation.  That's because the
   > exception handler doesn't "continue" at the initial
   > continuation of that thread.  Here are the relevant words of
   > SRFI 18:
   
   >
   >  Moreover, in this dynamic environment the exception handler
   >  is bound to the "initial exception handler" which is a unary
   >  procedure which causes the (then) current thread to store in
   >  its end-exception field an "uncaught exception" object whose
   >  "reason" is the argument of the handler, abandon all mutexes
   >  it owns, and finally terminate.
   >
   
   >The rationale is that, when an uncaught exception occurs in a
   >thread the thread is in bad shape and things have gone
   >sufficiently wrong that there is no universally acceptable way to
   >continue execution.  Executing after thunks could require a
   >whole lot of processing that the thread is not in a shape to do.
   >So the safe thing is to terminate the thread.  If the programmer
   >knows how to recover from an exception, then he can capture the
   >continuation early on, and install an exception handler which
   >invokes the continuation.  When the continuation is invoked the
   >after thunks will execute.

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 -gui option. In GUI-mode, the runtime system displays error messages in a message box and does some rudimentary command-line parsing.

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.

On Windows, `csc.exe` seems to be doing something else entirely.

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;

Macros

Where is `define-macro`?

In CHICKEN 4 and 5, the macro-expansion subsystem is now hygienic where old Lisp-style low-level macros are not available anymore. define-syntax can define hygienic macros using syntax-rules or low-level macros with user-controlled hygienic with explicit renaming macros. Translating old-style macros into ER-macros isn't that hard, see Macros for more information.

Why are low-level macros defined with `define-syntax` 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.

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:

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.
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 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:

 (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:

 (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-4 and posix fail to load?

When you try to import a built-in unit such as posix (which defines among other things, the (chicken posix) module), you may get the following error:

 #;1> (import (chicken posix))
 ; loading library posix ...
 Error: (load-library) unable to load library
 posix
 "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 'posix) 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 runtime option to your executable.

Optimizations

How can I obtain smaller executables?

XXX still valid?

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:

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

The following extended bindings are handled specially:

add1 any? arithmetic-shift atom? bignum? bit->boolean bitwise-and bitwise-ior bitwise-not bitwise-xor blob-size block-ref block-set! call/cc call-with-input-file call-with-output-file current-error-port current-thread error f32vector->blob/shared f32vector-length f32vector-ref f64vector->blob/shared f64vector-length f64vector-ref finite? fixnum? flonum? flush-output foldl foldr format fp* fp+ fp- fp/ fp<= fp< fp= fp>= fp> fpabs fpacos fpasin fpatan2 fpatan fpceiling fpcos fpexpt fpexp fpfloor fpinteger? fplog fpmax fpmin fpneg fprintf fpround fpsin fpsqrt fptan fptruncate fx*? fx* fx+? fx+ fx-? fx- fx/? fx/ fx= fx>= fx> fxand fxeven? fxior fxmax fxmin fxmod fxneg fxnot fxodd? fxrem fxshl fxshr fxxor identity infinite? list->string list->vector locative->object locative-ref locative-set! locative? make-record-instance nan? number-of-slots o pointer+ pointer->object pointer-f32-ref pointer-f32-set! pointer-f64-ref pointer-f64-set! pointer-s16-ref pointer-s16-set! pointer-s32-ref pointer-s32-set! pointer-s8-ref pointer-s8-set! pointer-u16-ref pointer-u16-set! pointer-u32-ref pointer-u32-set! pointer-u8-ref pointer-u8-set! pointer=? print* printf print s16vector->blob/shared s16vector-length s16vector-ref s16vector-set! s32vector->blob/shared s32vector-length s32vector-ref s32vector-set! s64vector->blob/shared s64vector-length s64vector-ref s64vector-set! s8vector->blob/shared s8vector-length s8vector-ref s8vector-set! signum sprintf string-append string->list sub1 substring substring-ci=? substring-index-ci substring-index substring=? u16vector->blob/shared u16vector-length u16vector-ref u16vector-set! u32vector->blob/shared u32vector-length u32vector-ref u32vector-set! u64vector->blob/shared u64vector-length u64vector-ref u64vector-set! u8vector->blob/shared u8vector-length u8vector-ref u8vector-set! vector->list

What's the difference betweem "block" and "local" mode?

In block mode, the compiler assumes that definitions in the current file are not visible from outside of the current compilation unit, so unused definitions can be removed and calls can be inlined. In local mode, definitions are not hidden, but the compiler assumes that they are not modified from other compilation units (or code evaluated at runtime), and thus allows inlining of them.

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).

If you have compiled code that contains a module definition, then executing the code will "register" the module to allow importing the bindings provided by the module into a running Scheme process. The information required to use a module is in this case embedded in the compiled code. Compiling another program that uses this (compiled) module is more difficult: the used module will not necessarily be loaded into the compiler, so the registration will not be executed. In this case the information about what bindings the compiled module exports must be separated from the actual code that executes at runtime. To make this possible, compiling a module can be done in such a manner that an "import library" is created. This is a file that contains the binding information of the module and we can use it to compile a file that refers to that module. An example can perhaps make this clearer:

 ;; my-module.scm
 
 (module my-module (...) ...)

 ;; use-my-module.scm

 (import my-module)
 ...

Compile the module and generate an import library for the "my-module" module:

 % csc -s my-module.scm -emit-import-library my-module

Compile the program that uses the module:

 % csc use-my-module.scm

Why is my program which uses regular expressions so slow?

The regular expression engine has recently been replaced by alex shinn's excellent irregex library, which is fully implemented in Scheme. Precompiling regular expressions to internal forms is somewhat slower than with the old PCRE-based regex engine. It is advisable to use irregex to precompile regular expressions outside of time-critical loops and use them where performance matters.

Garbage collection

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

Under CHICKEN's implementation policy, tail recursion is achieved simply by avoiding returns from function calls. 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-syntax v
  (syntax-rules ()
    ((_ 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 one of the breadline or linenoise eggs. Of these two, the breadline egg is recommended. After installing it, add the following to your ~/.csirc or equivalent file:

(import breadline)
(current-input-port (make-readline-port))

For a more extensive example with autocompletion and history file support, see the .csirc in the breadline egg distribution.

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).

How do I use extended (non-standard) syntax in evaluated code at run-time?

Normally, only standard Scheme syntax is available to the evaluator. To use the extensions provided in the CHICKEN compiler and interpreter, add:

(require-library chicken-syntax)

Extensions

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

You can just set the CHICKEN_INSTALL_REPOSITORY and CHICKEN_REPOSITORY_PATH environment variables. They should contain the path where you want eggs to be installed:

$ export CHICKEN_INSTALL_REPOSITORY=~/eggs/lib/chicken/5
$ export CHICKEN_REPOSITORY_PATH=~/eggs/lib/chicken/5
$ chicken-install -init ~/eggs/lib/chicken/5
$ chicken-install -p ~/eggs/ extensionname

In order to make programs (including csi) see these eggs, you should set CHICKEN_REPOSITORY_PATH when you run them. See the Extension tools/Changing repository location section of the manual for more information on that.

Alternatively, you can call the repository-path Scheme procedure before loading the eggs, as in:

(repository-path "/home/azul/eggs")
(import 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.

The repository needs to be initialized before use. See the documentation for the -init option to chicken-install, in Extensions.

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

Yes, just install them in a directory you can write to by using CHICKEN_INSTALL_REPOSITORY (see above).

Why does downloading an extension via `chicken-install` fail on Windows Vista?

Possibly the Windows Firewall is active, which prevents chicken-install from opening a TCP connection to the egg repository. Try disabling the firewall temporarily.

FAQ