[Haskell-cafe] Re: Small displeasure with associated type synonyms
haskell at list.mightyreason.com
Thu Mar 6 14:51:11 EST 2008
Tom Schrijvers wrote:
>> I tried lexically scoped type variables, but to no avail:
>> instance forall a b. (C a, C b) => C (a, b) where
>> type T (a, b) = (T a, T b)
>> val = (val :: T a, val :: T b)
> The problem is ambiguity. The type checker can't determine which val
> function to use, i.e. which dictionary to pass to val. Assume:
> instance C Int where
> type T Int = Int
> val = 0
> instance C Bool where
> type T Bool = Int
> val = 1
> Now, if you want some val :: Int, which one do you get? The one of C Int
> of C Bool? Depending on the choice you may get a different result. We
> can't have that in a deterministic functional language. Hence the error.
> Adding a type signature doesn't change the matter.
I don't see how your example explains this particular error.
I agree Int cannot be generalized to (T Int) or (T Bool).
I see Stefan's local type signature is not (val :: a) like your (val ::Int) but
(val :: T a) which is a whole different beast. And (T a) is the type that ghc
should assign here.
The C (a,b) instance wants val :: T (a,b), The T (a,b) is declared as "(T a, T
b)". The annotated val returns "(T a, T b)". One never needs the sort of Int
to (T Int) generalization.
So what is a better explanation or example to clarify why GHC cannot accept the
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