#4921: report ambiguous type variables more consistently
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Reporter: Saizan | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: 7.0.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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{{{
{-# LANGUAGE MultiParamTypeClasses #-}
module Amb where
class C a b where
f :: (a,b)
instance C Int Char where
f = undefined
{-
x = fst f
/home/saizan/snippets/Amb.hs:7:8:
Ambiguous type variables `a', `b' in the constraint:
`C a b'
arising from a use of `f' at /home/saizan/snippets/Amb.hs:7:8
Possible cause: the monomorphism restriction applied to the following:
x :: a (bound at /home/saizan/snippets/Amb.hs:7:0)
Probable fix: give these definition(s) an explicit type signature
or use -XNoMonomorphismRestriction
Failed, modules loaded: none.
-}
{-
y = fst f :: Int
/home/saizan/snippets/Amb.hs:21:8:
No instance for (C Int b)
arising from a use of `f' at /home/saizan/snippets/Amb.hs:21:8
Possible fix: add an instance declaration for (C Int b)
In the first argument of `fst', namely `f'
In the expression: fst f :: Int
In the definition of `y': y = fst f :: Int
Failed, modules loaded: none.
-}
}}}
Both x and y have the same problem, there isn't enough type information to
let the typechecker decide on an instance, so it seems they should produce
similar error messages.
In particular, the error for y is quite confusing since it can be
reasonably interpreted as saying there's no type b for which there's an
instance C Int b, which in fact is not true, so i think explicitly
mentioning the ambiguity like in the first message would help many to
understand the problem better.
I can see though that an "instance C Int b" could make sense, more often
than C a b, so maybe "Possible fix: add an instance declaration for (C Int
b)" should be conserved, even if it still has the problem of expressing
that the second argument needs to be a variable.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4921>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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