On Tue, Feb 23, 2021 at 06:14:59PM +0000, CASANOVA Juan wrote:
module DatatypeContextsExample where
import Data.Map import Data.Bifunctor
data Foo t = Foo (Map t t)
instance Functor Foo where fmap f (Foo m) = Foo (fromList (fmap (bimap f f) (toList m)))
This does not compile, because I am using toList and fromList, which require (Ord t). But I cannot really have Foo be a functor in this way without it. The thing is, every time I am going to use it, t is actually going to be Ord. But how do I tell Haskell to have this constraint?
You say that every time you are going to use it t is actually going to be Ord, but how does the compiler know that when it comes to type check fmap? After all, somewhere else you could write fmap (const getLine) :: Foo t -> Foo (IO String) and IO String is certainly not Ord. Tom