#3043: An unrelated definition monomorphizes a type even without the MR
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Reporter: Deewiant | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 6.10.1 | Severity: normal
Keywords: | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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The following code:
{{{
{-# LANGUAGE NoMonomorphismRestriction #-}
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances,
FunctionalDependencies #-}
class Id a b | b -> a where id' :: a -> b
instance Id [Char] [Char] where id' = id
class Fst a where fst' :: a -> String
instance Fst ([Char],a) where fst' = fst
data Void a = Void
void :: (String,a) -> Void a
void _ = Void
fst'' (a,b) =
let x = (id' a, b)
y = void x :: Void Int -- remove this line and the code compiles
in fst' x
}}}
Results in:
{{{
arst.hs:18:6:
No instance for (Fst (b, Int))
arising from a use of `fst'' at arst.hs:18:6-11
Possible fix: add an instance declaration for (Fst (b, Int))
In the expression: fst' x
In the expression:
let
x = (id' a, b)
y = void x :: Void Int
in fst' x
In the definition of `fst''':
fst'' (a, b)
= let
x = ...
y = ...
in fst' x
arst.hs:18:11:
No instance for (Id [Char] b)
arising from a use of `x' at arst.hs:18:11
Possible fix: add an instance declaration for (Id [Char] b)
In the first argument of `fst'', namely `x'
In the expression: fst' x
In the expression:
let
x = (id' a, b)
y = void x :: Void Int
in fst' x
}}}
It seems that the definition of `y` locks down the type of `x` somewhat
even though the monomorphism restriction is disabled. If we remove the
definition:
{{{
*Main> :t fst''
fst'' :: (Id t b, Fst (b, t1)) => (t, t1) -> String
}}}
To be completely honest I'm not sure whether the code should be accepted
or not, since `(Id t b, Fst (b, t1))` can't be satisfied. Only the fully
monomorphic signature `([Char], Int) -> String`, which is obtained with
the definition of `y` in place and the monomorphism restriction enabled,
works.
In any case, I think it's a bug that the results of the type check depend
on whether `y` is defined or not: surely that shouldn't matter at all, no
matter what the end result is.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3043>
GHC <http://www.haskell.org/ghc/>
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