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3043 An unrelated definition monomorphizes a type even without the MR Deewiant "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." bug closed normal Compiler (Type checker) 6.10.1 invalid Unknown/Multiple Unknown/Multiple