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

On Fri, Dec 13, 2013 at 5:57 PM, Mark Fredrickson 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

Wouldn't it make more sense to have fromInteger be `repeat . fromInteger`? This would make it an instance for ZipList, not list. You could even give a general instance for applicatives: instance (Num a, Applicative f) => Num (f a) where negate = fmap negate (+) = liftA2 (+) (*) = liftA2 (*) fromInteger = pure . fromInteger abs = fmap abs signum = fmap signum For ZipList this behaves like your instance, except fromInteger produces an infinite list like 'repeat . fromInteger'. The list instance instead computes all possiblities: ghci> [1,2,3] + [4,5,6,7] [5,6,7,8,6,7,8,9,7,8,9,10] Erik