[Haskell-cafe] polymorphic type

wenduan xuwenduan2010 at gmail.com
Tue Jul 5 15:52:49 EDT 2005

Dear all,

Suppose we have defined two functions as below:

case :: (a -> c,b -> c) -> Either a b -> c
case (f, g) (Left x) = f x
case (f, g) (Right x) = g x

plus :: (a -> b, c -> d) -> Either a b -> Either c d
plus (f, g) = case(Left.f, Right.g)

My question is regarding to the function signature of 'plus' , in its 
signature, does the 'a' in 'a -> b' and in 'Either a b', must be 
instantiated to the same object when the function is applied?E.g.,Either 
a b is instantiated to 'Either Char b', will the  'a' in  'a -> b' be 
instantiated to 'Either Char b'?Furthermore, are the two bs in 'a -> b' 
and 'Either a b' not conflicting?What I thought at first the signature 
of plus should be: plus :: (a -> c, b -> d) -> Either a b -> Either c 
d?Anyone know where I was wrong?


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