Hello,
I have a question regarding type family signatures. Consider the
following type family:
type family Fam a :: *
Then I define a GADT that takes such a value and wraps it:
data GADT :: * -> * where
GADT :: a -> Fam a -> GADT (Fam a)
and an accompanying unwrapper:
unwrap :: GADT (Fam a) -> (a, Fam a)
unwrap (GADT x y) = (x, y)
When Fam is declared using the first notation,
type family Fam a :: *
GHC HEAD gives the following error message:
Main.hs:9:21:
Couldn't match expected type `a' against inferred type `a1'
`a' is a rigid type variable bound by
the type signature for `unwrap' at Main.hs:8:20
`a1' is a rigid type variable bound by
the constructor `GADT' at Main.hs:9:8
In the expression: x
In the expression: (x, y)
In the definition of `unwrap': unwrap (GADT x y) = (x, y)
However, when Fam is declared as (moving the a to the other side of the
:: and changing it into *),
type family Fam :: * -> *
everything is ok. So, it seems to me that GHC HEAD considers both
signatures to be really different. However, I do not quite understand
the semantic difference in my example, other than that Fam needs to be
fully satisfied in its named type arguments. Note that GHC 6.10.3 does
not accept the latter signature for Fam since it requires at least one
index for some reason, that's why I'm using GHC HEAD.
Regards,
Thomas
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
