Functional MetaPost is a Haskell frontend to the MetaPost language by John
Hobby. Users write their graphics as Haskell programs, which then emit
MetaPost code that can be compiled into encapsulated PostScript files and
smoothly included into e.g. LaTeX.
A collection of useful examples how to use Functional MetaPost can be found
in the user's manual.
The document doesn't offer very much detail in terms of explanations, but the
code samples are quite helpful. Further documentation can be found in the
original thesis
that describes the implementation. The text is also available in
German.
Last but not least, there is a
tutorial
that offers many helpful examples (which is also available in
German).