SimpleFP
A series of implementations of a simple functional programming language, progressing from a very basic variant with no parametric types or polymorphism, through a parametric and polymorphic language, to a dependently typed language with various levels of user-friendliness.
The Core modules define the language proper. The Monadic modules use a monadic style for managing information. The Unification modules are a variant of the Monadic modules that use metavariables and unification instead of equality tests to allow a wider ranger of programs, especially where implicit arguments are present, as in the polymorphic variant.
You can try this out in cabal by doing cabal repl to load it up, then
Simple.Monadic.REPL.replFile "src/Demos.sfp"
This starts a REPL in the SimpleFP language, by loading and compiling the specified file (here just Demos.sfp). You can then test out the functions:
$> even Zero
True
$> odd (Suc (Suc Zero))
False