foo.idr

module Main

Semigroup b => Semigroup (a -> b) where
  f <+> g = \x => f x <+> g x

Monoid b => Monoid (a -> b) where
  neutral = const neutral

[all] Semigroup Bool where
  x <+> y  = x && y

public export
alphaNum : Char -> Bool
alphaNum = isDigit <+> isAlpha
-- this uses the a -> b instance, how can I tell it to then use [all] for b?

{-
Error(s) building file Test.idr: Test.idr:14:1--18:1:When elaborating right hand side of Main.alphaNum:
Test.idr:14:12--18:1:Can't solve goal ($resolved998 (({arg:5532} : Char) -> $resolved1048))
at line 14, character 12, file Test.idr
While processing right hand side of alphaNum at Test.idr:14:1--18:1:
Can't find an implementation for Semigroup (Char -> Bool)
-}