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{-# OPTIONS_GHC -fno-warn-redundant-constraints #-}
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
FlexibleInstances, FlexibleContexts, UndecidableInstances #-}
-- UndecidableInstances now needed because the Coverage Condition fails
-- !!! Functional dependency test. Hugs [Apr 2001] fails to typecheck this
-- Rather bizarre example submitted by Jonathon Bell
module ShouldCompile where
-- module Foo where
class Bug f a r | f a -> r where
bug::f->a->r
instance Bug (Int->r) Int r
instance (Bug f a r) => Bug f (c a) (c r)
f:: Bug(Int->Int) a r => a->r
f = bug (id::Int->Int)
g1 = f (f [0::Int])
-- Inner f gives result type
-- f [0::Int] :: Bug (Int->Int) [Int] r => r
-- Which matches the second instance declaration, giving r = [r']
-- f [0::Int] :: Bug (Int->Int) Int r' => r'
-- Wwich matches the first instance decl giving r' = Int
-- f [0::Int] :: Int
-- The outer f now has constraint
-- Bug (Int->Int) Int r
-- which makes r=Int
-- So g1::Int
g2 = f (f (f [0::Int]))
-- The outer f repeats the exercise, so g2::Int
-- This is the definition that Hugs rejects
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