Monday, February 07, 2011

Haskell Revelation -- Optimizing via refactoring

I have some code that lazily transforms a fairly large list of data (from an IO source) that must be search sequentially. Since it is lazy, the list isn't fully transformed until the first search. Since, I presume, a rather large stack of thunks are constructed instead, this first search takes a really, really long time. (It would be faster, I surmised, to strictly transform the list as it was being built, rather than lazily upon the first search).

I started playing with `seq` but couldn't quite get the strictness right -- the code represented some of my first attempts at Haskell.  So, I decided to refactor the code (replace naive tail recursion with maps, filters, folds, etc). I figured at this point I would be able to see more clearly how to avoid the lazy list.

Surprisingly, this refactoring was enough for the compiler to "do the right thing" and sped my application up significantly. What was the compiling doing here? Did it remove the laziness? Or did it just optimize the hell out of what I thought was a lazy vs strict problem?


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