Rewrite rules application prevented by type variable application (map id vs. map (\x -> x))

I’m moving the discussion from http://www.haskell.org/pipermail/glasgow-haskell-users/2013-January/023522.html (with reply at http://www.haskell.org/pipermail/glasgow-haskell-users/2013-January/023531.html) here, as a reminder for myself to work on it someday:

Short summary: a rule "map (\x -> x) = id" will not match "map id" because the latter has (in Core) an application of a type variable, despite the matcher looking through the definition of id. Doing beta-reduction (of type applications) in the matcher could fix this.

I’ll attach a test case.

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

Attached file 0001-Add-test-case-for-7611.patch (download).

Test case attached; you can also conveniently pull it via

git pull git://git.nomeata.de/ghc-testsuite.git T7611

Trac metadata

Trac field	Value
CC	pho@cielonegro.org → hackage.haskell.org@liyang.hu, pho@cielonegro.org

I think this may be related, but in this case I can't see what rule I could possibly write that would fire:

{-# LANGUAGE TypeFamilies #-}
type family Unit a :: *
type instance Unit Integer = ()

{-# INLINE [1] int #-}
{-# RULES "int" int = fromInteger :: Integer -> Int #-}

-- int :: (Integral i, Unit i ~ ()) => i -> Int -- doesn't fire
int :: (Integral i) => i -> Int -- fires
int = fromIntegral

Trac metadata

Trac field	Value
Version	7.6.1 → 7.6.2
TypeOfFailure	OtherFailure → RuntimePerformance

changed milestone to %7.8.1

changed milestone to %7.10.1

Moving to 7.10.1

removed milestone

Moving to 7.12.1 milestone; if you feel this is an error and should be addressed sooner, please move it back to the 7.10.1 milestone.

changed milestone to %8.0.1

Milestone renamed

closed

This is actually fixed, at least in 7.10, most likely by changeset:a27b2985/ghc.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

Great! Might you add a test case to make sure it stays fixed?

Thanks

Simon

mentioned in commit 97b47d27

added Pnormal label