#2440: Bad code with type families
-----------------------------------------+----------------------------------
Reporter: rl | Owner:
Type: run-time performance bug | Status: new
Priority: normal | Milestone: 6.10 branch
Component: Compiler | Version: 6.9
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Os: Unknown/Multiple
Architecture: Unknown/Multiple |
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Comment (by batterseapower):
(Repost for formatting reasons...)
Is there some reason the coercion for this case is so complicated? It
seems to me (with my admittedly limited knowledge of type coercions) that
we could improve it to:
{{{
(trans (sym (GHC.ST.NTCo:ST s GHC.Types.Int))
((sym (Foo.TFCo:R1:M s)) GHC.Types.Int))
:: STRep s Int ~ M (Vec s) Int
}}}
Here I have used the rewrite rules:
{{{
trans (x :: a ~ a) (x :: a ~ a) ==> x
sym (x :: a ~ a) ==> x
trans x (y :: a ~ a) ==> x
}}}
And optionally you could also do:
{{{
trans (sym x) (sym y) ==> sym (trans x y)
}}}
I don't think this helps fixing this bug at all, but it seemed a bit fishy
to me, and doing this improvement would at least help readability of Core.
Am I way off base here?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2440#comment:7>
GHC <http://www.haskell.org/ghc/>
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