The canonical way to get a perceptual hash is using the pHash library. In fact, we can get nearly the same performance in Haskell using hip and repa.
Here I'd like to show an example of runtime complexity, arising from a practical problem.
A question in compiler design is: what optimizations should a given compiler perform? Optimizations for functional languages in particular are not well-known; it is not obvious which optimizations will provide the greatest speedup on user code.
cpkg is now live on Hackage. It is a good deal less polished than I'd originally wanted, but it already gives impressive results (among them cross-compiling XMonad).
The "obvious" way to write a monadic zygomorphism is to look at the definition for an ordinary zygomorphism, namely
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