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SMLtoJs (pronounced "SML toys") is a compiler from Standard ML to JavaScript, which allows programmers to enjoy the power of Standard ML static typing, higher-order functions, pattern matching, and modules for programming client-side web applications.

SMLtoJs compiles all of Standard ML, including most of the Standard ML Basis Library. It also has good support for integrating with JavaScript.


  • All of Standard ML. SMLtoJs has support for all of Standard ML, including modules, pattern matching, higher-order functions, generative exceptions, etc.

  • Standard ML Basis Library support. SMLtoJs has support for most of the Standard ML basis library, including the following structures:

    Array2 ArraySlice Array Bool Byte Char CharArray CharArraySlice
    CharVector CharVectorSlice Date General Int Int31 Int32 IntInf
    LargeWord ListPair List Math Option OS.Path Pack32Big
    Pack32Little Random Real StringCvt String Substring Text Time
    Timer Vector VectorSlice Word Word31 Word32 Word8 Word8Array
    Word8ArraySlice Word8Vector Word8VectorSlice
  • JavaScript integration. SMLtoJs has support for calling JavaScript functions and for executing JavaScript statements.

  • Simple DOM access support and support for installing Standard ML functions as DOM event handlers and timer call back functions.

  • Optimization. All Modules language constructs, including functors, functor applications, and signature constraints, are eliminated by SMLtoJs at compile time. Moreover, SMLtoJs performs a series of compile time optimizations, including function inlining and specialization of higher-order recursive functions, such as map and foldl. Optimizations can be controlled using compile-time flags. As a result, SMLtoJs generates fairly efficient JavaScript code, although there are rooms for improvements; see below.

  • Compiling in the Browser. A version of SMLtoJs can be compiled by SMLtoJs itself, which leads to a proper browser hosted Standard ML compiler.

Online Demonstration

To see SMLtoJs in action, see the SMLtoJs homepage for links to online examples of compiled Standard ML and the Web browser hosted Standard ML compiler.

Getting the Sources

SMLtoJs compiles on Linux and Mac OS systems with MLton or MLKit. The SMLtoJs sources are hosted at Github. To get the latest sources, issue the following git command:

$ git clone smltojs

This command copies the sources to the directory smltojs.

Building SMLtoJs

To compile SMLtoJs from the sources (see above), simply type

$ cd smltojs
$ ./autobuild
$ ./configure
$ make smltojs
$ make smltojs_basislibs

If compilation succeeds, an executable file bin/smltojs should now be available.

How it Works

The SMLtoJs executable bin/smltojs takes as argument an sml-file (or an mlb-file referencing the sml-files and other mlb-files of the project) and produces an html file called run.html provided there are no type errors! The resulting html-file mentions the generated JavaScript files and a file prims.js, which contains a set of primitive JavaScript functions used by the generated code.

Hint: Adding the flag -o name as command-line argument to smltojs results in the file name.html being generated instead of run.html.

Testing that it Works

To compile and test the test programs, cd to the js/test directory and run make clean all:

$ cd js/test
$ make clean all

You can now start Firefox or Chrome on the generated html-files; the file all.html includes links to all the test files:

$ firefox all.html

The examples temp.html, counter.html, and life.html are the most interesting examples at the moment (more will come).

Compilation in the Browser

To build the browser-hosted compiler, proceed as follows:

$ cd ../../src/Compiler
$ SML_LIB=$(HOME)/smltojs/js ../../bin/smltojs -aopt

The last command should generate a file run.html which links to all necessary JavaScript sources. Executing

$ firefox run.html

will start up a simple IDE for the compiler.


There is a known issue with a bug in the following test (in some cases, the implementation pretty prints reals slightly different than suggested by the spec):

test13c: WRONG

There are plenty of possibilities for further improvements, including:

  • Functor in-lining, which may lead to improved execution speed.

  • Improved constant folding. Some features are implemented with the --aggressive_opt flag to smltojs (-aopt).

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.

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