On Sun, Sep 12, 2010 at 12:23 PM, Petr Prokhorenkov
I experienced a following problem while dealing with some text processing.
Thanks for the report and the test case. There's nothing wrong with your code - read on for details. You ran into one of the few functions in Data.Text that I copied straight over from the list implementation due to it not being used often. Unfortunately, that implementation constructs a new Text value (using cons) on every iteration through the loop, and as you might expect that's very slow even on tiny inputs, as it has quadratic behaviour. I've rewritten both strict and lazy mapAccumL and mapAccumR to use as much of the stream fusion machinery as possible. (By the way, there's an interesting fact behind why those functions started out life as they did: you can't write mapAccum functions using only stream machinery, due to their types, and the strict code is more work to write if you can't use the stream machinery. In the early days it just wasn't worth writing the more complex variants of them, as I had more pressing performance concerns at the time.) Where the old version of mapAccumL caused your test case to take 5 seconds to process an 11KB file (ouch!), with the rewritten version, your code can process an 81MB file in the same amount of time, without any changes to your code, and that O(n^2) behaviour is gone :-) text 0.8.1.0 is now up on hackage, with the fix included. Enjoy!