On 08/01/2012 21:13, Brandon Allbery wrote:
(Also, de facto I think it's already more or less been decided in
favor of type families, just because functional dependencies are (a) a
bit alien [being a glob of Prolog-style logic language imported into
the middle of System Fc] and (b) [as I understand it] difficult to
verify that the code in the compiler is handling all the potential
corner cases right [mainly because of (a)].
Without meaning to express an opinion either way about an issue I don't
understand...
Isn't Haskell doing some prolog-ish things anyway?
I thought the compiler must be doing unification to resolve type
inference within expressions. It's not a simple expression evaluation
problem (just evaluate the type rather than the value) because sometimes
you know the return type but not (yet) the argument types - type
information flows bottom-up and top-down through the same expression tree.
I could easily be mistaken, though. Looking at the similar
overload-resolution problem, Ada can resolve based on return types but
C++ cannot. Ada needs unification or something similar to resolve
overloading, whereas C++ just evaluates expressions for type instead of
value.
I can't now say for sure where I picked up the idea that Haskell needs
unification to resolve type inference, but I've had some odd error
messages which seem to confirm that belief - I assume because the
mistake doesn't cause an immediate conflict, instead causing an indirect
conflict somewhere else in the larger expression.
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