[Haskell-cafe] Odd behavior with Num instance for lists (was: f^n for functional iteration)

Mark Fredrickson mark.m.fredrickson at gmail.com
Fri Dec 13 16:57:44 UTC 2013

On Fri, Dec 13, 2013 at 8:59 AM, Antonio Nikishaev <me at lelf.lu> wrote:

> http://hackage.haskell.org/package/NumInstances

The previous exchange prompted me to whip up a Num instance for lists:

    instance Num a => Num [a] where
        negate = fmap negate
        (+) = zipWith (+)
        (*) = zipWith (+)
        fromInteger x = [fromInteger x]
        abs = fmap abs
        signum = fmap signum

It mostly behaves how one would expect:

    let a = [1,2,3]
    ghci> let b = [4,5,6,7]
    ghci> a + b
    ghci> 1 + a

I was wondering whey `1 + a` succeeds. At first I thought it could be the
`fromInteger` definition, but this explanation were true, I should be able
to add integers and doubles freely, which I can't:

    ghci> fromInteger (1::Integer) + (1.0::Double)
    ghci> (1::Integer) + (1.0::Double)

    Couldn't match expected type `Integer' with actual type `Double'
    In the second argument of `(+)', namely `(1.0 :: Double)'

Thanks for enlightening me.

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