"Functional Pearl: Polytypic Unification", Patrik Jansson and Johan Jeuring, Journal of Functional Programming
The core of the unification algorithm is written in Haskell (using the interpreter Hugs) and the polytypic part is written in the Haskell extension PolyP. The full unification code (download 6k) is available together with the August 1998 version of the PolyP compiler (download 99k). (A version of Hugs, ghc or hbc is needed for compilation.)
If you download both files the following commands will compile and run the test program:
gunzip polyp.tar.gz tar xf polyp.tar gmake -C polyp hugs gunzip PU.tar.gz tar xf PU.tar gmake -C PU PolyP=../polyp/bin/hugspolyp
For more information on the latest developments on polytypic programming, see the authors Polytypic Programming page.