On 01/14/2017 02:35 AM, Graham Gill wrote:
Do I need two different versions of f, one for Bounded a and one for non-Bounded a? Is there a more elegant way to take care of this problem? I don't know much about all of the type magic available in GHC.
You probably want your own typeclass instead of Enum, even if it means generating a bunch of very boring instances of it for the types you want "f" to work on. The fact that "pred" should throw a runtime error on "minBound" is a documented fact of the Enum typeclass, and you should stay far far away from such things in your own code. Besides that, there's a weird interaction between the semantic meaning of Enum and Bounded. For example, here's a perfectly valid enumeration of boolean values: True, False, True, False, ... In your case it would be fine to have (pred False) == True, but instead you get a runtime error thanks to the Bounded instance. So being Bounded rules out some otherwise valid (and fine for your purposes) Enum instances. Your "f" should also work on a singleton type: ghci> data Foo = Foo deriving (Eq,Show) ghci> instance Enum Foo where toEnum _ = Foo; fromEnum _ = 0; ghci> groupConsecutive [Foo,Foo,Foo,Foo] [[Foo,Foo,Foo,Foo]] But any Bounded instance for Foo would mess that up. Basically, the pre-existing Enum instances aren't exactly what you want.