Really bad code for single method dictionaries?
batterseapower at hotmail.com
Fri Mar 27 04:18:49 EDT 2009
2009/3/26 Jason Dusek <jason.dusek at gmail.com>:
> 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.
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