Hi,
Seems that Haskell allows to specify "dummy" type variables in a
declaration of a type synonym, which do not appear in its right-hand
side. This can lead to interesting effects, which appears differently
in GHC and Hugs. I would like to know, what behavior is correct
according to the haskell 98 report.
1)
------------------------------------------
type F a = Int
class A a where
foo :: A b => a (F b)
------------------------------------------
GHC - OK
Hugs - Illegal type "F b" in constructor application
2)
------------------------------------------
type F a = Int
class A a where
foo :: F a
instance A Bool where
foo = 1
instance A Char where
foo = 2
xs = [foo :: F Bool, foo :: F Char]
------------------------------------------
GHC:
M.hs:14:6:
Ambiguous type variable `a' in the constraint:
`A a' arising from a use of `foo' at M.hs:14:6-8
Probable fix: add a type signature that fixes these type variable(s)
M.hs:14:21:
Ambiguous type variable `a1' in the constraint:
`A a1' arising from a use of `foo' at M.hs:14:21-23
Probable fix: add a type signature that fixes these type variable(s)
Hugs: [1,2]
Thanks,
Vladimir
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