fundeps.hs

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}
module Lib where

class C a b | a -> b

instance C () Int

class Foo a where
    foo :: C a b => a -> b -> IO ()

instance Foo () where
    foo () int =
        print int

{-
/home/matt/Projects/fundeps/src/Lib.hs:13:9: error:
    • Could not deduce (Show b) arising from a use of ‘print’
      from the context: C () b
        bound by the type signature for:
                   foo :: forall b. C () b => () -> b -> IO ()
        at src/Lib.hs:12:5-7
      Possible fix:
        add (Show b) to the context of
          the type signature for:
            foo :: forall b. C () b => () -> b -> IO ()
    • In the expression: print int
      In an equation for ‘foo’: foo () int = print int
      In the instance declaration for ‘Foo ()’
   |
13 |         print int
   |         ^^^^^^^^^

-}