#1900: Type families with class constraints: type-checker loops
------------------------+---------------------------------------------------
Reporter: h.holtmann | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.9
Severity: normal | Keywords:
Testcase: | Architecture: x86
Os: Linux |
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The following program:
{{{
{-# LANGUAGE TypeFamilies, FlexibleContexts #-}
class (Eq (Depend s))=> Bug s where
type Depend s
trans :: Depend s -> Depend s
instance Bug Int where
type Depend Int = ()
trans = (+1)
check :: (Bug s) => Depend s -> Bool
check d = d == trans d
}}}
runs into a type-checker loop
{{{
GHC-Bug.hs:1:0:
Context reduction stack overflow; size = 20
Use -fcontext-stack=N to increase stack size to N
`ic :: {(Bug Int, Eq (Depend Int)) => Num ()}'
arising from the type signature for `trans' at GHC-
Bug.hs:24:2-13
`ic :: {(Bug Int, Eq (Depend Int)) => Num ()}'
arising from the type signature for `trans' at GHC-
Bug.hs:24:2-13
<snip>
}}}
Of course, this program should not compile, but the type-checker should
not loop.
compiler: 6.9.20071105
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1900>
GHC <http://www.haskell.org/ghc/>
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