On Mar 8, 2008, at 22:06 , Thomas Hartman wrote:
A minor issue: I had a question if I could make the type signatures for Enum instances less verbose by doing something like type BoundedEnum = (Bounded a, Enum a) => a... I tried, and commented out my attempt as it wouldn't type check. Guidance appreciated.
Nope. Uses of "type" declarations are self-contained; if you do type BoundedEnum = (Bounded a, Enum a) => a myFunc :: BoundedEnum -> BoundedEnum -> BoundedEnum each instance of BoundedEnum in the declaration of BoundedEnum refers to a distinct a. More technically, the declaration expands to: myFunc :: (forall a. (Bounded a, Enum a) => a) -> (forall a. (Bounded a, Enum a) => a) -> (forall a. (Bounded a, Enum a) => a) which is a rather useless declaration (I think the only inhabitant of that type is _|_). And yes, this is annoying. I think you might be able to do this as a typeclass instead, at the expense of having to insert an instance declaration for each type. (You will have to use an extension if you want to declare instances for types such as Int. I think.) class (Bounded a, Enum a) => BoundedEnum a where -- empty instance BoundedEnum MyType a where instance BoundedEnum Int where -- requires FlexibleInstances? -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH