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{-# LANGUAGE Rank2Types #-}
-- A classic test for type inference
-- Taken from "Haskell and principal types", Section 3
-- by Faxen, in the Haskell Workshop 2003, pp88-97
module ShouldCompile where
class HasEmpty a where
isEmpty :: a -> Bool
instance HasEmpty [a] where
isEmpty x = null x
instance HasEmpty (Maybe a) where
isEmpty Nothing = True
isEmpty (Just x) = False
test1 y
= (null y)
|| (let f :: forall d. d -> Bool
f x = isEmpty (y >> return x)
in f y)
test2 y
= (let f :: forall d. d -> Bool
f x = isEmpty (y >> return x)
in f y)
|| (null y)
|
