#4398: Panic with FlexibleContexts and rewrite rules
---------------------------------+------------------------------------------
Reporter: batterseapower | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.12.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Compile-time crash
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I wanted a rewrite rule that fired if one of the type variables was an
instance of Ord. To make that work, I tried something like this:
{{{
{-# LANGUAGE FlexibleContexts #-}
{-# RULES "suspicious" forall (x :: a) y. f (x :: Ord a => a) y = g x y
#-}
{-# NOINLINE f #-}
f :: a -> a -> Bool
f x y = True
g :: Ord a => a -> a -> Bool
g = (<)
main = print $ f 2 1
}}}
Which generates a rewrite rule like this:
{{{
"suspicious" ALWAYS
forall {@ a $dOrd :: Ord a x :: a y :: a}
f @ a x y
= g @ a $dOrd x y
}}}
Naturally, $dOrd isn't present on the left hand side so we get a panic.
Is there any way to write this kind of rule without inducing a panic? I
suspect that there isn't, but would like to be proven wrong.. sometimes
you can optimise better if you can e.g. see that a type is Orderable.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4398>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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