"To Be Or Not To Be Standard ML" ;-)

Version 2.0.0

What is it?

HaMLet is a faithful and complete implementation of the Standard ML programming language (SML'97). It aims to be

The implementation is intended to be as direct a translation of the language formalisation found in the Definition of Standard ML [1] as possible, modulo bug fixes. It tries hard to get all details of the Definition right. The HaMLet source code

HaMLet can perform different phases of execution — like parsing, elaboration (type checking), and evaluation — selectively. In particular, it is possible to execute programs in an untyped manner, thus exploring a universe where even ML programs "can go wrong".

It should be emphasized that HaMLet is by no means a development system, but has been solely written with the aforementioned goal of experimentation in mind. Interpretation is highly inefficient (since it is a direct implementation of the semantic rules) and error messages are rather basic. However, HaMLet is able to bootstrap itself.

As a byproduct, the HaMLet documentation contains a comprehensive list of all known bugs and `grey areas' in the current version of the SML language definition, which may be interesting on its own.

What's new?

Release 2.0 (2013/10/10) brings a major revamp of the internal AST representation. In particular, elaboration now stores its results in the AST, which should make HaMLet more useful as an experimental compiler front-end. As a proof of concept, the release also integrates a simple compiler to JavaScript.

The most significant changes are:

See the change log for more details.


The HaMLet sources are available as a tarball, zipfile or Debian package:


For questions, comments and bug reports please contact the author at

Feedback is always welcome.

Successor ML

There also is a special "HaMLet S" that incorporates proposals for Successor ML (sML). It represents a testbed and sort of a personal vision of where sML might go have gone. Its most interesting features are:


See changes for a version history. Note that HaMLet S is still based on HaMLet 1.3.

Other Implementations of Standard ML

SML implementations more suitable as proper development systems are:

HaMLet evolved as a byproduct of the Alice project, and owes much of its existence to the first version of the ML Kit, which took a very similar approach.


  1. Robin Milner, Mads Tofte, Robert Harper, David MacQueen.
    "The Definition of Standard ML" (Revised).
    The MIT Press, 1997

  2. Emden Gansner, John Reppy.
    "The Standard ML Basis Library".
    Cambridge University Press, 2004

Andreas Rossberg - last modified 2013/10/12