| > g = \x::(Int,Bool) -> let-type (a,b) = (Int,Bool) in e
| >
| > But notice that the RHS of a pattern-matching let-type is
| statically
| > guaranteed to have the right shape. So I don't allow
| >
| > let-type (a,b) = c in ...
| >
| > (where c is a type variable).
|
| Really? I disagree. Making pattern type sygnatures perform
| unification of the variable with the supplied signature is
| useful for resolving ambiguities, and backward-compatible
| (valid programs will remain valid), and doesn't allow
| "unwanted generality" to cause trouble (when an expression
| has a more general type than needed).
I was meaning in the translation into System F of the program.
Under my proposal (= yours) if one writes
f (x::(a,b)) = e
then f is forced to have type
f :: (T1, T2) -> ...
for some T1, T2. In saying that let-type can't pattern-match against
a type variable c, I'm just saying that f cannot have type
f :: forall c. c -> ....
I think I just didn't express myself well enough.
Simon
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