#5051: Typechecker behaviour change
---------------------------------+------------------------------------------
Reporter: igloo | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.0.2
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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From: http://www.haskell.org/pipermail/glasgow-haskell-
users/2011-March/020116.html
The behaviour has changed (7.0.1 accepts it, 7.0.2 doesn't), but I'm not
sure which behaviour is right.
I've put a cut-down (but not standalone) version of the offending module
below. I believe the relevant steps are then:
{{{
The ct application means we need an instance for:
Cast (ResidueI (Pol (ResidueE (UPol k))))
(Pol (ResidueE (UPol k)))
Need: Cast (ResidueI (Pol (ResidueE (UPol k))))
(Pol (ResidueE (UPol k)))
Have: instance LinSolvRing a => Cast (ResidueI a) a
Now need: LinSolvRing (Pol (ResidueE (UPol k)))
Have: instance EuclideanRing a => LinSolvRing (Pol a)
Now need: EuclideanRing (ResidueE (UPol k))
Have: instance (EuclideanRing a, Ring (ResidueE a) )
=> EuclideanRing (ResidueE a)
Now need: EuclideanRing (UPol k)
Have: instance Field a => EuclideanRing (UPol a)
Now need: LinSolvRing (UPol k) (from the EuclideanRing class
constraints)
Have: instance EuclideanRing a => LinSolvRing (UPol a) (Pol2_.hs:95)
And: instance (LinSolvRing (Pol a), CommutativeRing a)
=> LinSolvRing (UPol (Pol a))
A different instance should be used depending on whether or not
k = Pol x
(for some x).
}}}
With flags:
{{{
-XTypeSynonymInstances -XUndecidableInstances -XFlexibleContexts
-XFlexibleInstances -XMultiParamTypeClasses -XOverlappingInstances
-XRecordWildCards -XNamedFieldPuns -XScopedTypeVariables -fcontext-
stack=30
}}}
{{{
module T_cubeext (cubicExt) where
import Prelude (undefined)
import DPrelude (ct)
import Categs (ResidueE)
import SetGroup ()
import RingModule (Field, FactorizationRing)
import VecMatr ()
import Fraction ()
import Pol (Pol, UPol)
import Residue (ResidueI)
import GBasis ()
cubicExt :: forall k . (Field k, FactorizationRing (UPol k))
=> k -> ()
cubicExt _ = undefined
where unE :: ResidueI (Pol (ResidueE (UPol k)))
unE = undefined
kToE :: Pol (ResidueE (UPol k)) -> ResidueI (Pol (ResidueE (UPol
k)))
kToE = ct unE
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5051>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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