Suggestion: map2 variant that builds the cartesian product

[srijs]

Hi!

As someone coming to Elm from Haskell, I was quite surprised by the fact that map2 doesn't work coherently with concatMap.

I would have expected this to hold:

List.map2 f xs ys = List.concatMap (\a -> List.map (f a) ys) xs

In other words, map2 building the cartesian product of both lists, instead of zipping. The reason I expected it is because I was looking at map2 and concatMap as the Elm variants of <*> and >>=.

I actually think that a zipping map2 is more approachable, so I like the way things currently are. However, I was wondering whether you would be open to adding the other functionality as well, under a different name (and possible extend the docs to make that clearer)?

[jvoigtlaender]

First, please read https://github.com/elm-lang/core/blob/master/CONTRIBUTING.md#adding-new-functions.

So, such a function, if considered for addition, should first go into some *-extra package. And indeed, it already is: http://package.elm-lang.org/packages/circuithub/elm-list-extra/3.10.0/List-Extra#lift2.

About documentation: Maybe the current documentation for the List module in the core package should prepare against the surprise you encountered? You could make a suggestion for how to change the current documentation as shown at http://package.elm-lang.org/packages/elm-lang/core/3.0.0/List#map2. For that, please open a new issue or pull request.

[jvoigtlaender]

First, please read https://github.com/elm-lang/core/blob/master/CONTRIBUTING.md#adding-new-functions.

So, such a function, if considered for addition, should first go into some *-extra package. And indeed, it already is: http://package.elm-lang.org/packages/circuithub/elm-list-extra/3.10.0/List-Extra#lift2.

About documentation: Maybe the current documentation for the List module in the core package should prepare against the surprise you encountered? You could make a suggestion for how to change the current documentation as shown at http://package.elm-lang.org/packages/elm-lang/core/3.0.0/List#map2. For that, please open a new issue or pull request.