Really bad code for single method dictionaries?

[Neil Mitchell]

Hi Jason,

While experimenting with Uniplate I found that 1-member dictionaries
were faster than N element dictionaries - which seems to run against
what you see in the comment. 1-member dictionaries being cheaper does
make sense as then instead of passing a tuple containing functions,
you can pass the direct function, and save yourself a (cheap) selector
call at every use.

Thanks

Neil

On Thu, Mar 26, 2009 at 11:29 PM, Jason Dusek <jason.dusek at gmail.com> wrote:
>  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.
>
> --
> Jason Dusek
>
>
>
>  |...stream fusion code...|
>  http://www.cse.unsw.edu.au/~dons/code/streams/list/Data/Stream.hs
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