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Comment by Sharlin | Hacker News Reader

The equivalent of `flatMap` is not `flatten`. Semantically `flatMap` is equivalent to `(comp flatten map)` – hence the name! – but Clojure does offer it as a single function named `mapcat` (”map and catenate”).

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> Semantically `flatMap` is equivalent to `(comp flatten map)`

With the caveat that `flatten` is recursive and can handle a mix of collections and non-collections, while `mapcat` is non-recursive and every value returned by the mapcatted function must be a collection or nil.

(Identity returns its argument.)

    (flatten [1 [2 3] [[4 5 6]]])
    > [1 2 3 4 5 6]

    (mapcat identity [1 [2 3] [[4 5 6]]])
    > IllegalArgumentException Don't know how to create ISeq from: java.lang.Long

    (mapcat identity [[1] [2 3] [[4 5 6]]])
    > (1 2 3 [4 5 6])

    (flatten [nil [1 2 3]])
    > (nil 1 2 3)

    (mapcat identity [nil [1 2 3]])
    > (1 2 3)

Thanks, I wasn’t aware that flatten is recursive.

That's sort of implied by the name. A non-recursive flatten wouldn't really flatten much, and would be called join, or concatenate.

The flatten functions I know from statically typed languages (eg. Rust, Scala) only flatten one level. So flatMap is actually more or less equivalent to (flatten . map).

Interesting. My experience is the opposite - in dynamically typed languages like Erlang, Clojure or Common Lisp, flatten is understood to flatten lists, and flatmap is something else entirely - from what I gather, it's Scala that made the weird choice of naming a concatenation function `flatten`, thus creating `flatMap`[0], and then people started borrowing the name.

From this I provisionally conclude that "flatten" being non-recursive is an artifact of Scala confusing things; not an unusual thing in my experience with this language, but this time it seems to have gotten out of hand, instead of being contained within Scala ecosystem.

[0] - Even Haskell calls this `concatMap`, see [1].

[1] - https://stackoverflow.com/questions/49843262/where-does-the-...

Flatten is a steam-roller and should be used wisely: it recursively flatten a sequential collection. Most of the time this is not what you want. Most of the rest of the time when you think that’s what you want it’s a code smell telling you that the shape of your items has gone out of control. And there’s the rare occurrence where you can really use it.

Oops, indeed. Didn’t realize that Clojure flatten is recursive.

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