[Haskell-cafe] checking types with type families
simonpj at microsoft.com
Thu Jul 1 13:54:11 EDT 2010
| Well, from looking at the documentation, it looks like I could maybe
| use a type family if I could write:
| class (DerivedOf a ~ derived) => Typecheck a derived where
That's the right idiom yes. But see my message of a few minutes ago... It's neater still to remove the 'derived' parameter altogether, which you can do with uni-directional fundeps. Thus instead of
class Typecheck a derived where
op :: a -> derived
class Typecheck a where
type Derived a
op :: a -> Derived a
| Presumably then I could combine functions 'Typecheck a derived'
| constraints without getting lots of "Could not deduce (Typecheck a
| derived3) from the context (Typecheck a derived13)" errors. I'm only
| guessing though, because it looks like it's not implemented yet as of
Correct. These equality superclasses will work in 6.14 and (in a few weeks) in the HEAD, but not in 6.12.
But you may not need them if you have unidirectional fundeps.
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