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STklos (version 1.50)

Erick Gallesio <eg(at)>


STklos is a free Scheme system compliant with the languages features defined in R7RS. The aim of this implementation is to be fast as well as light. The implementation is based on an ad-hoc Virtual Machine. STklos can also be compiled as a library and embedded in an application.

The salient points of STklos are:

  • an efficient and powerful object system based on CLOS providing
    • Multiple Inheritance,
    • Generic Functions,
    • Multi-methods
    • an efficient MOP (Meta Object Protocol)
  • a simple to use module system
  • a full tower of numbers implementation, as defined in R5RS,
  • easy connection to the GTK+ toolkit,
  • a Perl compatible regular expressions thanks to the PCRE package,
  • easily extensible with its ScmPkg mechanism,
  • it implements properly tail recursive calls.


STklos is the successor of STk, a Scheme interpreter tightly connected to the Tk toolkit, that I have developed since September 1993.

STklos is very similar to STk, for the user, but very different in he implementation. Everything has been rewritten and STklos uses now a compiler to a byte-code machine, rather than direct interpretation. As a consequence, it is a lot faster than STk. STklos is also better designed and should be easily embedded in an application (it was not completely the case with STk).

Whereas STk used the Tk toolkit, STklos uses the GTK+ graphical toolkit. STklos is not completely compatible with STk but it should be easy to port old STk graphical applications (provided that they use objects). The GTK+ extension can be loaded at run time and does not require a special interpreter as it was the case with STk.

July 2019 The 1.3x versions try to be more R7RS compliant. All the R5RS functions which have been extended in R7RS are now conform to R7RS (for instance assoc and member accept now an optional parameter which is a compare function, vector->list accepts the start and end parameters, ...)

June 2020 The 1.40 version continues to add R7RS traits to STklos.

August 2020

The 1.50 version has been ported on various architectures (in particular on `small" systems running on 32 bits little as well as big endian) and a port on Android. This version brings also the support of 23 new SRFIs. For a complete list of supported SRFIs, see

SRFI Support

Current version of STklos supports some SRFIs (a complete list of implemented SRFIs is available in the file SUPPORTED-SRFIS in the distribution). The goal is to support as much as possible final SRFIs. Any help on implementing other SRFIs on STklos is welcome.


There are two recommended IDEs that can be used with STklos:

Supported architectures

STklos current version works on

  • GNU/Linux 4.x and 5.0 (64 bits).
  • Mac OS X 10.13 High Sierra
  • Win32 (using WSL - Windows Subsystem for Linux)

The previous version of STklos (1.10) was known to work on the following architectures:

  • GNU/Linux 1.6.x (i386/gcc 32 and 64bits)
  • Mac OS X 10.5+ (i386/gcc 32 and 64 bits)
  • FreeBSD 8.x (i386/gcc 32 bits and 64 bits)
  • Win32 (using Cygwin 1.7.x)

Very old versions of STklos have run on various architectures. All these ports are pretty old now, but if a port worked a day on a given architecture, it should be not too hard to port recent version of STklos on it. The old ports were effective on:

  • Win-NT/Win-XP (i386/gcc(Cygwin))
  • GNU/Linux >= 2.2 (i386/gcc and alpha/gcc)
  • FreeBSD 4.2 (i386/gcc)
  • Mac OS X (ppc & Intel/gcc)
  • Solaris 8 (ultrasparc/gcc)
  • Irix 6.5.20 (Octane, mips R10,000)
  • Open Darwin 7.0 (PPC/gcc)

More informations on supported platforms is available in the PORTING-NOTES file in the distribution.

STklos Installation

See the file ./ for instructions on how to build STklos (a more complete document is available in ./INSTALL

If you want to build or run a Docker image of STklos, you can find some information in the ./etc/Docker/

Project Home

STklos home page is is located at