The importance of reasoning about and refactoring programs is a central tenet
of functional programming.  Yet our compilers and development toolchains only
provide rudimentary support for these tasks.  To address this, we have
implemented HERMIT, a toolkit enabling informal but systematic transformation
of Haskell programs from inside the Glasgow Haskell Compiler's optimisation
pipeline.  With HERMIT, users can experiment with optimisations and equational
reasoning, while the tedious heavy lifting of performing the actual
transformations is done for them.

HERMIT provides a transformation API that can be used to build higher-level
rewrite tools.  In this talk I will describe one HERMIT application---a
read-eval-print shell for performing transformations using HERMIT.  I will
also demonstrate using this shell to prototype an optimisation on a specific
example, and discuss our initial experiences and remaining challenges.