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The MLKit is a compiler toolkit for the Standard ML language, including The MLKit with Regions, which features a native backend for the x64 architecture, based on region inference, and SMLtoJs, which features a JavaScript backend targeting web browsers. The two compilers share the same frontend and compilation management scheme.

The MLKit covers all of Standard ML, as defined in the 1997 edition of The Definition of Standard ML and supports most of the Standard ML Basis Library.

Test Statistics and Benchmarking

CI Benchmarking


Under macOS, MLKit is available through Homebrew: Just execute brew install mlkit. Under Linux, you may download the latest binary tgz-distribution of MLKit from

Once downloaded and unpacked, execute make install from within the top-directory of the unpacked distribution. You may install MLKit in a directory different from /usr/local/mlkit by instead typing PREFIX=myinstallpath make install.

General Features

  • Compiles all of Standard ML. The MLKit compiles all of Standard ML, including Modules, as specified by the Definition of Standard ML. The MLKit also supports most of the Standard ML Basis Library.

  • Compiles large programs. The MLKit compiles large programs, including itself, around 80.000 lines of Standard ML plus the Standard ML Basis Library. The support for ML Basis Files makes it easy to compile a program with different Standard ML compilers. Currently, both MLton and the MLKit supports the concept of ML Basis Files. The MLKit works well together with smlpkg, a generic package manager for Standard ML libraries and programs.

  • Documentation is available. Man-pages and general documentation is available from the MLKit home page.

MLKit with Regions - The x64 Native Backend

This version of the compiler is based on region inference and has the following features:

  • An x64 native backend (works with Linux and macOS).

  • Memory allocation directives (both allocation and deallocation) are inferred by the compiler, which uses a number of program analyses concerning lifetimes and storage layout. The MLKit compiler is unique among ML implementations in this respect.

  • A comprehensive guide on Programming with Regions in the MLKit is available, which also demonstrates how to create memory profiles of program executions using the supplied region profiler and how to interact with C programs.

  • Region inference may be augmented with reference-tracing garbage collection to achieve better memory behavior.

SMLtoJs - The JavaScript Backend

This version of the compiler generates efficient JavaScript, primarily for executing Standard ML code in the browser. There is also an online version of SMLtoJs, which makes it possible to write, compile, and execute Standard ML code in a web browser.

The Barry Backend

The repository also includes the sources for Barry, a Standard ML source-to-source compiler that eliminates modules, using static interpretation, and generates optimised Core-language Standard ML code.

License and Copyright

The MLKit compiler is distributed under the GNU Public License, version 2. See the file MLKit-LICENSE for details. The runtime system (/src/Runtime/) and libraries (basis/) is distributed under the more liberal MIT License.

Compilation Requirements

To compile, install, and use the MLKit, a Linux box running Ubuntu Linux, Debian, gentoo, or similar is needed. The MLKit also works on macOS and has also earlier been reported to run on the FreeBSD/x64 platform, with a little tweaking.

To compile the MLKit, a Standard ML compiler is needed, which needs to be one of the following:

MLton >= 20051202:

$ mlton
MLton 20051202 (built Sat Dec 03 04:20:11 2005 on pavilion)

If a version prior to 20201023 is used, you may need to adjust the mlton-flags setup in the file Makefiledefault.

A working MLKit compiler >= 4.3.0:

$ mlkit -V
MLKit version 4.3.0, Jan 25, 2006 [X86 Backend]

Moreover, gcc is needed for compiling the runtime system and related tools.


After having checked out the sources from Github, execute the command:

$ ./autobuild

Now, cd to the toplevel directory of the repository and execute the appropriate set of commands:

Compile with MLton alone (Tested with 3Gb RAM):

$ ./configure
$ make mlkit

Compile with existing MLKit (Tested with 1Gb RAM):

$ ./configure --with-compiler=mlkit
$ make mlkit

If you later want to install the MLKit in your own home directory, you should also pass the option --prefix=$HOME/mlkit to ./configure above.

For binary packages, we use

$ ./configure --sysconfdir=/etc --prefix=/usr

Pre-compile Basis Library and Kit-Library

Execute the following command:

$ make mlkit_libs

Bootstrapping (optional - works with 1Gb RAM)

This step is optional. If you want the resulting executable compiler to be bootstrapped (compiled with itself), execute the command:

$ make bootstrap && make mlkit_libs

Be aware that this step takes some time.

Installation after Compilation

For a system-wide installation of the MLKit, including installation of man-pages and tools, execute the command:

$ sudo make install

For a personal installation, with --prefix=$HOME/mlkit given to ./configure, execute the following command:

$ make install

Making a Binary Package

To build a binary package, execute the command

$ make mlkit_x64_tgz

This command leaves a package mlkit-X.Y.Z-x64.tgz in the dist/ directory. For building a binary package, the installation step above is not needed and the bootstrapping step is optional. The binary package includes both the MLKit with Regions compiler (i.e., the mlkit executable) and SMLtoJs (i.e., an executables named smltojs).

Try It

To test the installation, copy the directory /usr/share/mlkit/kitdemo to somewhere in your own directory, say $HOME/kitdemo:

$ cp -a /usr/share/mlkit/kitdemo $HOME/kitdemo
$ cd $HOME/kitdemo
$ mlkit helloworld.sml

The MLKit should produce an executable file run:

$ ./run
hello world

Trying Without Installing

You can run mlkit without installing it, but you should then point the environment variable SML_LIB at the build directory (which contains the basis and the lib directories) whenever you run mlkit. E.g:

$ SML_LIB=$PWD bin/mlkit

More Information

See the MLKit home page for information about related papers, etc.

General documentation for the MLKit is located in the directories doc/mlkit and man/man1. License information is located in the file doc/license/MLKit-LICENSE.

Comments and Bug Reports

The MLKit has a number of known bugs and limitations. To file a bug-report, create an issue at the Github page.

Appendix A: Directory Structure of the Sources


Appendix B: Quick Compilation and Installation Guide

We assume that MLton >= 20051202 is installed on the system as described above.

After having checked out the sources from Github, execute the command:

$ ./autobuild

To compile the MLKit, execute the following commands:

$ ./configure
$ make mlkit
$ make bootstrap
$ make mlkit_libs

The make bootstrap command is optional.

To install the MLKit and related tools, execute:

$ sudo make install

See the section "Try It" above to test the installation.

Appendix C: Displaying Region Flow Graphs with VCG

The VCG tool can be used to show region flow graphs.