#6002: GHC 7.4+ thinks class instance is incoherent, 7.0.4 disagrees
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Reporter: heisenbug | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.4.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: GHC rejects valid program | Testcase:
Blockedby: | Blocking:
Related: |
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Load this file into GHCi:
http://omega.googlecode.com/svn/mosaic/TypeMachinery.lhs?r=1086
With v7.4 (and HEAD) I get:
{{{
GHCi, version 7.5.20120410: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling TypeMachinery ( TypeMachinery.lhs, interpreted )
TypeMachinery.lhs:78:23:
Overlapping instances for Show (Nat' (Plus n n1))
arising from a use of `Hide'
Matching instances:
instance Show (Nat' a) -- Defined at TypeMachinery.lhs:24:3
There exists a (perhaps superclass) match:
from the context (Show (Nat' n))
bound by a pattern with constructor
Hide :: forall (a :: * -> *) n. Show (a n) => a n ->
Hidden a,
in an equation for `+'
at TypeMachinery.lhs:78:5-10
or from (Show (Nat' n1))
bound by a pattern with constructor
Hide :: forall (a :: * -> *) n. Show (a n) => a n ->
Hidden a,
in an equation for `+'
at TypeMachinery.lhs:78:14-19
(The choice depends on the instantiation of `n, n1'
To pick the first instance above, use -XIncoherentInstances
when compiling the other instance declarations)
In the expression: Hide
In the expression: Hide $ plus a b
In an equation for `+': (Hide a) + (Hide b) = Hide $ plus a b
Failed, modules loaded: none.
}}}
With 7.0.4 all compiles (and runs) correctly.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6002>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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