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

On Fri, Dec 13, 2013 at 8:57 AM, Mark Fredrickson < mark.m.fredrickson@gmail.com> wrote:

On Fri, Dec 13, 2013 at 8:59 AM, Antonio Nikishaev 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 [5,7,9] ghci> 1 + a [2]

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

<interactive>:30:17: Couldn't match expected type `Integer' with actual type `Double' In the second argument of `(+)', namely `(1.0 :: Double)' ...

Thanks for enlightening me.

`1 + a` works for the same reason `1 + (1.0::Double)` works: type inference. `1 + a` in this instance is equivalent to `(1 :: Num [Integer]) + a` which should compile down to something equivalent to `fromInteger 1 + a`. -bob