2009/3/26 Jason Dusek
I was reading the stream fusion code today and came across a comment stating that single element dictionaries interacted poorly with GHC's optimizer:
class Unlifted a where
[...] expose [...]
-- | This makes GHC's optimiser happier; it sometimes produces really bad -- code for single-method dictionaries -- unlifted_dummy [...]
A cursory search on GHC's Trac shows no corresponding bug; is this no longer a problem? A small problem? I would like to know more about it.
I'm not sure if there is a bad interaction with rewrite rules (which I assume was the complaint) but single method dictionaries tend to be faster because they are represented by a coercion in the Core language, which means that packing and unpacking the dictionary is free at runtime. Cheers, Max