On Sun, Aug 31, 2008 at 5:11 PM, David House
Here's an example of such a concern: you write the following:
module Amb where import Data.Map
foo = map
What is the type of `foo'?
Exactly the same as now: it has no type: amb.hs:4:6: Ambiguous occurrence `map' It could refer to either `Prelude.map', imported from Prelude or `Data.Map.map', imported from Data.Map at amb.hs:2:0-14 On the other hand, this module would no longer fail to compile:
module Amb where import Data.Map
foo :: (a -> b) -> [a] -> [b] foo = map
which right now gives the same error message as above, but with my proposal would successfully compile, with foo = Prelude.map. As Philippa said, the naive implementation is: 1) Attempt to typecheck with all permutations of ambiguous names 2a) If zero permutations typecheck, type error. 2b) If multiple permutations typecheck, the program is ambiguous (current behavior) 2c) Otherwise, exactly one typechecks successfully. Specialize the ambiguous names to the chose permutation and continue compilation. It is a conservative extension of the language, because any program that successfully compiles now has no change: there are no ambiguous names in any currently compiling Haskell program. But it accepts some additional programs that have ambiguities, by choosing the only result that typechecks. If there is more than one possible type for foo, the program is still ambiguous and therefore should not compile. -- ryan