[Discussion moved from Haskell to Haskell-Cafe] Hi, Regarding - "lazy overlap resolution" aka unique instances Well, if there's only instance which is not exported, then you can use functional dependencies. Assume class C a instance ... => C t Internally, use class C a | -> a instance ... => C t Furthermore, there seems to be an issue that has been overlooked so far. - Providing sufficient type annotations Recall Lennart Augustsson writes:
Here is a small puzzle.
-- The following generates a type error: f :: Char -> Char f c = let x = g c in h x
Lennart "magically" inserted a type annotation to resolve the ambiguity.
-- But this definition does not: f :: Char -> Char f c = let x :: Bool x = g c in h x
Clearly, there are a number of ways to resolve the ambiguity. The Chameleon type debugger assists the user in identifying which locations are responsible. Here's the Chameleon type debugger error message generated for the unannotated function f temp.ch:2: ERROR: Inferred type scheme is ambiguous: Type scheme: forall b. C b => Char -> Char Suggestion: Ambiguity can be resolved at these locations b: f :: Char -> Char f c = let x = g c ^ ^ in h x ^ ^ (The ^ correspond to highlightings of the Chameleon type debugger) This may help the user to realize where and which annotations are missing. E.g., each of the following annotations (there are more possibilities) will do the job. f :: Char -> Char f c = let -- x :: Bool (1) x = g c in h x -- h (x::Bool) (2) -- (h::Bool->Char) x (3) Martin