[Haskell-cafe] Simple type-class experiment turns out not so simple...
sh006d3592 at blueyonder.co.uk
Mon Jan 9 10:37:06 CET 2012
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
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
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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