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It's probably worth augmenting the tutorial to mention that you're using the Hashmap (Data.HashMap.Strict) in the Unordered-Containers package, rather than any other ones, as its the most performant of the versions available and thus the one folks should use (amongst the hashmap implementations)

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Absolutely, I did experiment with http://hackage.haskell.org/package/hashtables, but did not notice enough of a difference in performance to decide it was worth using in this case, especially with the extra visual overhead of the state monad. I had expected it to be much faster, but I would probably chalk that up to me making mistakes in how to use it (iirc I wrote my own insertWith).

I think the hashetable package has a bit of a different performance / use case profile than the functional hash map data structure. Also I believe you need to spend more effort tuning which type of hashtable and your choice of hash function based upon your use case/ blend of operations. Some of the hashtable options regress to linear time insertions and lookups under certain use cases and load factors

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