Hacker News Reader

Comment by krat0sprakhar | Hacker News Reader

The OCaml version (for 2 lists) is almost as good as clojure

    List.map2 ( + ) a b;;

However, for 3 or more lists, Clojure definitely wins (since there's no variadic equivalent of map in OCaml) -

    let x = List.map2 ( + ) a b in List.map2 ( + ) x c;;

Which got me wondering, is there a particular reason why only a `map2` exists in OCaml?

View on HN

Not surprisingly, the sum of two lists is just as nice in Haskell:

    zipWith (+) a b

... and things get a little bit hairy with three lists:

    zipWith3 (\x y z -> x + y + z) a b c

... which is still nice but for four lists we get to the same point as OCaml:

    let a' = zipWith (+) a b in zipWith3 (\x y z -> x + y + z) a' c d

I guess in both OCaml and Haskell the user is expected to define zipWith4, zipWith5, etc. (or map in OCaml's case) if he fells the need to use them.

There is probably a simple and elegant way to get variadic-like behavior in Haskell by folding list of list, or playing with Foldable and Sum, but it's a little bit to early in the morning for me to generalize it.

You can also write:

    liftA2 (+)

    (+) <$> a <*> b

if you have the right type-class instance.

You could also go all the way and declare a Num a => Num [a] typeclass, with (+) = zipWith (+).

See https://hackage.haskell.org/package/base-4.8.2.0/docs/Data-L...

Reply on news.ycombinator.com