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{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE OverlappingInstances #-}
 
class IsList a where
  isList :: a -> Bool
 
instance IsList a where
  isList x = False
 
instance IsList [a] where
  isList x = True
 
main = print (isList 42) >> print (isList [42])  
 

[1 of 1] Compiling Main             ( prog.hs, prog.o )

prog.hs:13:22:
    Ambiguous type variable `a0' in the constraints:
      (Num a0) arising from the literal `42' at prog.hs:13:22-23
      (IsList a0) arising from a use of `isList' at prog.hs:13:15-20
    Probable fix: add a type signature that fixes these type variable(s)
    In the first argument of `isList', namely `42'
    In the first argument of `print', namely `(isList 42)'
    In the first argument of `(>>)', namely `print (isList 42)'

Standard output is empty