UPDATE: I've worked this into a library in a private project, that abstracts over Free and CPS'd F for efficiency. If you'd be interested in using this, let me know and I can release a package.

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An example Haskell implementation of the "free transformers" approach to building modular programs by composing layers of Free interpreters.

This was adapted from a combination of:

• http://mpickering.github.io/posts/2014-12-20-closed-type-family-data-types.html
• http://degoes.net/articles/modern-fp
• http://degoes.net/articles/modern-fp-part-2
• https://github.com/notxcain/onion-architecure
• https://gist.github.com/jdegoes/97459c0045f373f4eaf126998d8f65dc

In this example, the code does not (yet) go so far as to abstract over the "computational context" (i.e. sequential/Free vs parallel/FreeAp) -- everything is currently sequential.

If you have any tips/ideas/fixes, please let me know!