[Haskell-cafe] Ambiguous types for collection keys
weeksie at twelvestone.com
Wed Apr 12 15:18:47 EDT 2006
> Well, if you get an "ambiguous type variable" error, you probably (I
> think) need to add some type annotations. For example:
> class Foo a where
> foo :: a
> bar :: a -> String
> Evaluating bar foo will result in an error, but bar (foo :: Integer)
> will work just fine.
The problem is that I get an incoming value which is a key of some
sort. There's no way of knowing what type that value is supposed to be
until I compare it with the schema from the above example. where I _am_
adding type annotations.
coerceIndex f (Schema _ SInt SPtr _) (r,hdl,o,hdr) = f
coerceIndex f (Schema _ SStr SPtr _) (r,hdl,o,hdr) = f
When I try to add type annotations I get a complaint from the
typechecker that says (In the case of the above example) Expected type:
Integer, Inferred Type: PackedString.
Is the alternative to write different "select" methods for each key
type (selectInt, selectPS, ...)? God I hope not, that would be a bit
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