Polymorphic variable used in different contexts haskell

Question

I have a following piece of Haskell code:

foo :: Num a => (a -> a) -> Either Integer Double -> Either Integer Double
foo f x = case x of
  Left i -> Left $ f i
  Right d -> Right $ f d

It doesn't compile with error "Couldn't match type Integer with Double". I understand that type of f is computed as Integer -> Integer and Double -> Double in Left and Right expressions respectively, creating a collision.

How can I change this function, so that correct version of f would be used each time?

YellPika · Accepted Answer · 2014-01-13 00:24:38Z

6

You need RankNTypes.

{-# LANGUAGE RankNTypes #-}

foo :: (forall a. Num a => a -> a) -> Either Integer Double -> Either Integer Double
foo f x = case x of
    Left i -> Left $ f i
    Right d -> Right $ f d

answered Jan 13, 2014 at 0:24

YellPika

2,9022 gold badges29 silver badges31 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Polymorphic variable used in different contexts haskell

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related