[Haskell-beginners] type of zipWithPlus

[Russ Abbott]

Thanks. I tried the first two, which worked.

Previously I had tried:

> let (zipWithPlus :: Num a => [a] -> [a] -> [a]) = zipWith (+)

<interactive>:1:5:
    Illegal signature in pattern: (Num a) => [a] -> [a] -> [a]
        Use -XScopedTypeVariables to permit it

I tried setting -XScopedTypeVariables. When I ran it again I got a different
error message:

Prelude> let (zipWithPlus :: Num a => [a] -> [a] -> [a]) = zipWith (+)


<interactive>:1:5:
    Illegal polymorphic or qualified type:
      forall a. (Num a) => [a] -> [a] -> [a]
    Perhaps you intended to use -XRankNTypes or -XRank2Types
    In a pattern type signature: (Num a) => [a] -> [a] -> [a]
    In the pattern: zipWithPlus :: (Num a) => [a] -> [a] -> [a]
    In a pattern binding:
        (zipWithPlus :: (Num a) => [a] -> [a] -> [a]) = zipWith (+)



-- Russ Abbott
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  Professor, Computer Science
  California State University, Los Angeles

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On Thu, Sep 30, 2010 at 12:48 PM, Daniel Fischer
<daniel.is.fischer at web.de>wrote:

> let zipWithPlus xs ys = zipWith (+) xs ys
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