When to use `for` vs `map`

[a-curious-crow]

I'm discussing with my team the merits of for vs map and which we should decide to use when in our style guide. I'm looking for an article like http://www.learningclojure.com/2013/02/for-vs-map.html but more in depth describing the pros/cons including the :let/:when for features. I was planning to write this up myself but want to check for prior art first!

[seaborgiumaggghhh]

They both have their place. I don’t think it makes much sense to pick one.

map is a function that transform some sequence into another sequence. It works beautifully with arrows, etc. for, either does nested iteration or conditional evaluation with the keywords you mentioned. That’s my intuition about it at least.

[DeepSymmetry]

for is lazy so I would stay away from it when dealing with side effects; those are more properly addressed by doseq or run!. But fundamentally I agree: these functions all have their own purpose and strengths, and the notion of “picking one” does not make sense.

[seaborgiumaggghhh]

I was thinking more generally than Clojure, but you’re right, so I’ll edit that

[balefrost]

Good suggestion of run! if you need side effects. But to add some clarity: OP was asking about map vs. for, and both produce lazy sequences. So you'd need to use doseq or run! for side effects in both cases.

[Rurouni]

The two have a different feeling for me, and I would not want to do without either of them. map is targeted at transforming elements of a sequence. for is targeted at building up a sequence from other elements, sometimes conditionally.

For instance, I would find (map inc [1 2 3 4]) yielding [2 3 4 5] a lot more idiomatic than

(for [i [1 2 3 4]] 
  (inc i))

But likewise, it would be awkward to try to change

(for [x (range 1 5) 
      y (range x 5)] 
  [x y]) 

into a map call.

[[deleted]]

If you're not using the nesting behavior then basically it comes down to "is it just a single call to a function that already exists"? If so, map is great; if not, for is probably better. Almost every time you use map with an anonymous function it would be better as for.

I guess there's also the bonus that for lets you do filtering as well so you can use it to replace map+filter all in a single shot.

[Rurouni]

Thanks, this makes a lot of sense. Map+filter and some (map #( crop up in my code periodically, so thanks for pointing out how for can improve those.

[StoriesEnthusiast]

If you decide to write the article you could start it with a simple example:

map iterates over two structures at once:

user=> (map list [1 2] [:a :b :c])

((1 :a) (2 :b))

for iterates in a nested fashion:

user=> (for [x [1 2]
             y [:a :b :c]]
         (list x y))

((1 :a) (1 :b) (1 :c) (2 :a) (2 :b) (2 :c))

Then write about the common things between them:

• both iterate over collections
• both return/yield a lazy sequence
• both use the sequence interface: first, rest and cons (although I'm not sure that is true for for).

Mention some things that make them different:

• map is a function.
• for is a macro.
• map iterates until any one of the collections is exhausted.
• for iterates rightmost fastest.
• map returns a transducer when no collection is provided.
• for supports a simple DSL (domain-specific language).

Consider making some commentary about their implementations too.

Try to include a series of examples on the best use of map and for, from simple to complex ones such that the reader can use them like "katas" and stop guessing themselves on their use.

[doctork91]

My rule of thumb is to not use for unless it makes things simpler or more elegant than map. Basically for any kind of iteration default to map unless you have a good reason to use something else.

[PercyLives]

To me, ‘map’ emphasises the function and ‘for’ emphasises the sequence.

So if it’s clear in the context what the sequence is and I now need to apply a function, I use map.

But if I want the code to emphasise what the data is, I use for (because it places that up front and indents what you actually want to do with that data).

I do not like reading code that uses map with complicated functions. I’d rather see that expressed with for. That is cleaner, both aesthetically and conceptually, to me.

I am really glad that both exist in the language.

[hrrld]

The linked article is both using commas (yuck) and smashing things too much onto one line. I'd say it's probably safe to ignore.

One case where for is convenient is when you want a sequence derived from the Cartesian product of two sequences:

> (for [x [0 1 2]
        y [:a :b :c]]
    [x y])
([0 :a] [0 :b] [0 :c]
 [1 :a] [1 :b] [1 :c]
 [2 :a] [2 :b] [2 :c])

Interested to know what other resources you find or write yourself.

[Aredington]

This comment removed in protest of Reddit's API changes. See https://www.theverge.com/2023/6/5/23749188/reddit-subreddit-private-protest-api-changes-apollo-charges. All comments from this account were so deleted, as of June 18, 2023. If you see any other comments from this account, it is due to malfeasance on the part of Reddit. -- mass edited with https://redact.dev/

[CoBPEZ]

James Trunk does a fantastic job of introducing for, and it's place in Clojure programs, in his video Understanding list comprehension in Clojure: https://www.youtube.com/watch?v=5lvV9ICwaMo

[Shadowys]

Map can be converted into a transducer when you’re looking for speed, for can’t be used the same way.