[Haskell-cafe] DatatypeContexts / alternative

[Tom Ellis]

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