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Comment by brundolf | Hacker News Reader

FP is about avoiding side-effects; forEach() can only cause side-effects, so it's not really functional at all, and yet it's superficially designed to blend in with the actual functional array methods like map().

IMO it's the worst of both worlds and there's virtually never a good reason to use it. If you're going to write a side-effect-y loop, a classic for-loop (or more often, a for..of loop) helps to emphasize that the code is side-effect-y.

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While I agree on the side-effect side of things, you really shouldn't use a classic for loop when forEach suffices.

forEach emphasizes you are operating on each element of the loop in a semi-independent fashion and you explicitly don't have access to any kind of intermediary index/iterator variable.

When you "drop down" to a classic for loop you're telling the reader that you need access to the index/iterator for some reason (and there are plenty of valid reasons for that).

If you only use classic for loops then the reader has to be constantly vigilant you're not doing any index/iterator shenanigans. It's just higher "cognitive load"

I think `for element of list` nowadays starts to become a realistic choice (without a transpiler) for that: https://caniuse.com/?search=for...of

As an added benefit, `for...of` works with async, unlike forEach.

If I would be tempted to use forEach but can’t because async, I usually don’t want for…of semantics, though. I probably want something like

  await Promise.all(collection.map(thing => some_async_function(thing)));

instead of:

  for thing of collection {
    await some_async_function(thing);
  }

The index and the collection iterated over is passed as the 2nd/3rd parameters in JavaScript, so technically you still have access to them, though in the majority of cases people will omit these parameters

> The index and the collection iterated over is passed as the 2nd/3rd parameters in JavaScript,

Yeah, the “map and friends pass extra parameters that are usually ignored” plus the “lots of JS functions optionally take extra parameters that are usually unused” makes it so easy to think of a function as a one-arg function when its really one plus multiple optionals, use it bare in a map, and get stung by it.

>forEach emphasizes you are operating on each element of the loop in a semi-independent fashion and you explicitly don't have access to any kind of intermediary index/iterator variable.

I disagree with this point, specifically for JavaScript the for...of syntax is more pleasant to read and also does not infer that any index access is gonna be done. And as a plus, it is easier to be used with async/await.

oh yikes sorry. I didn't realize JavaScript is so idiosyncratic. i was just kinda thinking from my c++ bubble i guess

All most programmers will need is a classic for-loop. For of, forEach and any other kind of iterator is really not necessary and adds to the language bloat that just confuses less experienced developers.

Actually, all most programmers need is either `map` or `reduce`. A classic `for` loop is too low-level for most people’s needs.

I find it really difficult to believe that there are developers out there that never need to be able to break or continue in a loop. What exactly would they be developing?

I’m not sure where you got “never” from in my comment.

I believe it was gotten from the ambiguities of English - when you say "all most programmers need is " it could mean all that is needed most of the time but probably the more usual meaning would be that a great number of programmers (most programmers - greater than 50% of programmers), will never need anything but `map` or `reduce`. If the second meaning is taken then this means as a corollary that these developers never need to be able to break or continue in a loop.

Of course if it were assumed they never need anything but the higher level methods: map, reduce, some, every etc. - then they would probably use some or every if they wanted to break.

> this means as a corollary that these developers never need to be able to break or continue in a loop.

No it doesn’t.

I don't understand this no it doesn't, the logic I outlined can be boiled down to

If I only need to do X

And X cannot do Y

Then I do not need to do Y

-----------------------------

If I need to do Y

And X cannot do Y

Then I need to do something other than X

-------------------------

Give your answer of no it doesn't please be so kind as to outline the logical reasoning that you are envisioning?

on edit: added line between logical statements.

Unless I've misunderstood you, you said that, for example, `continue` cannot be implemented in terms of `map`, which simply isn't true. It's trivial.

I thought the original statement was break or continue, I was actually focusing on break? As far as I know you don't break in map, you can return null but not break the loop itself.

on edit: although I almost never break in a loop, I use some other construct instead.

on second edit: maybe I should say iteration instead of loop but will let it stand.

It's not clear to me what you're trying to achieve. Sorry.

out of the edit window, evidently was typing quickly, but would not use every for implementing a break like functionality on an array - some could do it in some instances or maybe find.

I suppose filter could maybe be hacked to do it in some cases dependent on implementation, if the implementation determined that it was impossible to ever return true for any of the items remaining in an array that implementation might effectively break the array processing however every way I can think of doing that would be artificial, horrible, and probably full of side effects.

Come on, you just said that most people only need map or reduce, which implies they never need to break or continue.

Regardless of how you choose to interpret my comment, it is the case that both `break` and `continue` are literally never necessary, as those two constructs can be implemented in terms of `map` and `reduce`.

ok, how do you implement a break of the iteration in map in JavaScript? I believe it cannot be done, you believe it can? Teach me.

There is no sensible way to `break` the iteration while mapping. That's not what `map` is for. In fact, there's very little reason for anyone to need `break` ever. The use of `break` implies an effectful procedure rather than a pure function, so at this point you're not really doing Functional Programming.

Let me know what it is you're trying to achieve — at a high level, not in terms of specific implementation details — and I can show you how this might be expressed without using anything as low-level as a `for` loop.

I'm not asking help with programming anything, and it seems somewhat insulting that somehow you have managed to turn multiple people's observations that you can't break in a map and so most programmers probably need something more than map or reduce into a request for tutelage - especially when you've spent a lot of time saying the impossible (implementing break in map) was trivial before switching to observing it was not sensible to do - which I take as meaning an admission of the aforementioned impossibility.

so to clarify - I am not asking for your help in solving my programming problems. You made a response to another poster asserting that >all most programmers need is either `map` or `reduce`

that poster said >I find it really difficult to believe that there are developers out there that never need to be able to break or continue in a loop.

you said >I’m not sure where you got “never” from in my comment.

I, in my normal long-winded way gave you the benefit of the doubt that you meant something else but advised the previous poster had assumed you meant more than 50% of programmers will not ever need other array methods than map or reduce and hence was confused because you cannot break or continue with those (with map and reduce together you can implement something that has the same end effect as continue but it is not 100% the same of course)

but for some reason you just wanted to keep saying you were right and others were wrong even though the clear meaning of everyone's text was - you can't break in a map.

And you said:

>Regardless of how you choose to interpret my comment, it is the case that both `break` and `continue` are literally never necessary, as those two constructs can be implemented in terms of `map` and `reduce`.

frankly exasperated I said

>ok, how do you implement a break of the iteration in map in JavaScript? I believe it cannot be done, you believe it can? Teach me.

now admittedly here you may think I am confused and need help, but really I am saying you are wrong in a nice way when you say break can be implemented in map. But then you say:

>There is no sensible way to `break` the iteration while mapping. That's not what `map` is for.

right, what everyone's been telling you from the beginning.

>Let me know what it is you're trying to achieve — at a high level, not in terms of specific implementation details — and I can show you how this might be expressed

Thank you for your kindness but it should be obvious from the whole conversation nobody is asking you for programming help here, they are just saying your initial statement of most programmers only need map and reduce was overly bold.

As to why someone might want to break a loop, generally you do that when you have a long array - say 10000 items (don't bother telling me that 10000 is not a long array, I know, but a long array is often used as meaning an array requiring a lot of processing and that is partially determined by what you need to do with that array, if that is not good enough for you arbitrarily add 0s to the array length until you feel you have a long array) and have a number of conditions that can cause you to only have to process a number of them which probably in that case you would use find (because often in such a case you are processing an array to find one item in it) or if for some reason you needed to drop down to a lower level I would prefer to use while instead of a for loop because semantically I think while indicates to anyone reading the code without going into the loop - hey Bryan doesn't expect to have to look at every element of this array!

BUT ALL THAT DOESN'T MATTER - because the subject matter of this long discussion was you saying most programmers only need map and reduce and someone asked why do you think most programmers will NEVER need to break. And you wanted to know why they assumed that you thought most programmers would NEVER need to break because evidently you felt you never said anything remotely like that.

hey man -

i don't think you really listened to the other guy.

i think he's saying that there's no need for break/continue with FP

because you simply approach the problem from a different angle

it's a different paradigm

that's probably why he asked for an example (not to belittle you)

i might do a filter before a map to recreate a lot of the functionality of a continue or break

and when I wrote/write OOP I might avoid break/continue because these things can be leaky

ok, maybe I'm too aggressive lately, but the only reason to do a break is to stop iterating over the array. You can easily achieve the output of using break using filter, but you cannot achieve the behavior because filter goes over every item in your array.

Probably the most common reason you want to break is that you found what you were looking for so find is what you would use.

so to make an example that would not be reasonable for find:

you have a long array of objects, you need to display 5 of the 'interesting' objects out of this array. What constitutes 'interesting' is determined by some relatively complicated logic on each item in the array. Since you do not know how many items in this array are interesting you want to process all items until you have 5 items. When you have 5 items you want to stop processing items therefore you break, if your first 5 items have the property of being 'interesting' you just saved a lot of work in your application and things feel zippy so you want to do that (because your application maybe needs the help at this point)

It is obviously, as all examples when one does not need something at the very moment, somewhat artificial - but I would say it is not unthinkable.

However, as I made clear earlier, I am not arguing for the necessity of break - if I wanted to do what I just described I would probably use while unless the syntax became unwieldy - I dislike break.

The only reason I got into this is because someone said they didn't believe most programmers wouldn't ever need break (which can be used to stop iteration of an array) and the guy who said all most programmers needed was map and reduce (both of which iterate over every item of an array) then said they never said most programmers wouldn't need break.

If the language implements them as lazy iteration, then I think take_until would be effectively equivalent to break, with early exiting behavior.

List.where(…).map(…).take_until(…)

maybe, but that seems a weird way to say you can do it.

> If the language implements them as lazy iteration

JS doesn't, though.

> then I think take_until would be effectively equivalent to break

JS also doesn't have take_until.

>JS also doesn't have take_until.

I just assumed it was a new method coming out soon! Although when I was thinking of how I would make a method I just thought an until method that iterated all elements of an array outputting them until it got the return from the callback that was passed as the iteration stopper - default value null.

  const findInterestingItems = (arr, isInteresting, num, i=0) =>
    num == 0 || i >= arr.length
    ? []
    : isInteresting(arr[i])
      ? [arr[i], ...findInterestingItems(arr, isInteresting, num-1, i+1)]
      : findInterestingItems(arr, isInteresting, num, i+1)

I don't see where you used map? You made a recursive function? the argument was clearly stated - person says all most programmers need is map and reduce on arrays. I agree that by recursing you also have found another way not to use break but you have evidently had to NOT use map and reduce to do it?

on edit: also how will that recursion perform on the large array we've supposed as being used if it turns out that we don't get our interesting items in early parts of array?

ok someone showed a way you can achieve break like functionality with map https://news.ycombinator.com/item?id=30306256 by mutating the array parameter of the map.

> You can easily achieve the output of using break using filter, but you cannot achieve the behavior because filter goes over every item in your array.

Because JS’s map and filter provide the index and array as arguments, you can, in fact, achieve the behavior of break. If you need the original array after the iteration, you need to apply the map or filter to a copy, though, because the way to achieve the behavior of break involves modifying the array by deleting all the items after the current one, which stops the iteration.

yes, you showed this earlier https://news.ycombinator.com/item?id=30306256 by mutating the array parameter of the map, but I made my wrong statement before reading your example.

I guess it's a failure of imagination on my part not to have realized that I could modify the array length in that way, or really the failure of imagination was not seeing any good reason for sending the original array in as a parameter to map - when I saw that in documentation I thought what a weird idea!

a better idea is to first use filter() to obtain a collection you can then map() over. So then you dont need to continue or break, the offending elements of the collection were already filtered out.

But then you're processing the whole list, probably defeating the point of breaking.

i don't understand what you mean. Are you referring to performance or implementation details?

that's a performance optimization. you have other performance optimizations available often, with tradeoffs.

FP is a higher level paradigm so you might have to trade off with performance too

>FP is a higher level paradigm so you might have to trade off with performance too

Then it can hardly he said be to unequivocally be better, can it.

that's like saying high level languages cannot be unequivocally better then writing assembly, not doing your own memory management is bad, etc...

Functional programming is a higher level abstraction. If the underlying implementation is not well implemented/optimized then you could have bad performance while the opposite is also true.

>that's like saying high level languages cannot be unequivocally better then writing assembly,

Yes, it is like saying that. At least, assuming by "cannot be" you mean "is not".

>not doing your own memory management is bad

No, it's not like saying that. Note how differently those claims are phrased.

> ok, how do you implement a break of the iteration in map in JavaScript?

If you want something that behaves like:

  array.map(x => {
    if (x > 2) { break; } // can't really use break here!
    else { return x; }
  });

you do this:

  [...array].map((x,i,a) => {
    if (x > 2) { a.splice(i); }
    else { return x; }
  });

(if you don’t need the array later, you can just use “array” instead of [...array]; the latter is just used because the break effect is achieved by mutating the array to delete all the elements subsequent to the one you are working, terminating the iteration, and copying the array initially means you don’t stomp the original.)

huh, that's actually interesting and horrifying I guess, I knew the third argument of map was the original array but I had never seen a usage of it - I guess mutating an array while working on it goes against instinct for just about everyone.

so I guess I was wrong, you can get the same functionality of break in map, albeit in a way that, right at this moment, makes me uneasy.

I basically rely on map 80% of the time for iteration. I do have to spend some mental cycles on deciding where to go if I needed to break or skip elements. That was until I discovered the beautiful .flatMap() trick of returning an empty array to skip or exclude an index item.

Is it bad to use map?

What are you struggling with when trying to skip elements?

> Is it bad to use map?

No.

Whether to use a for loop or for of or forEach or map().filter() etc

flatMap takes care of a lot of that

Why are you using a “trick” to skip an element in some structure when mapping a function over it?

For posterity (and forgive me if this JavaScript syntax is inaccurate; I don't write it so much these days)…

    xs.map(a => shouldBeSkipped ? a : f(a))

They don't want to keep the unmapped value in the resulting collection at all. flatMap allows for removal of an element in one traverse, unlike filter+map with eager behaviour.

Then why not just use a fold?

Sure, there's many ways to skin a cat. Given the option, I wouldn't choose either of these, and instead use something like filterMap[1] which I think conveys intent better than a fold or flatMap.

Btw, I'm assuming that the original map questioner wasn't solely using flatMap for side-effectful iteration, which reading again, I'm a bit suspicious about.

1: https://gcanti.github.io/fp-ts/modules/Array.ts.html#filterm...

I see people use JavaScript Array.map a lot when they aren't doing anything with the returned array, which I find kind of irritating.

So if you're using map sensibly, no there's nothing wrong. But I don't like the dogma I read a lot which is "use map always" because it leads to inexperience developers using it in the wrong circumstances.

Does JS have something like array.iter?

> Actually, all most programmers need is either `map` or `reduce`.

You actually don't need `map`, you can easily implement that using `reduce` too ;)

I'm well aware of that, but I don't think that's a good idea.

There is utility in communicating whether you're working with a catamorphism vs a homomorphism. Code is written for humans, after all.

Of course, that wasn't meant seriously. That's why i like `for_each` (or `iter` or whatever it's called) as 'third' function too, because it makes it clear you're doing side-effects.

Nobody needs reduce, you’re just making code make less sense. If you’re using reduce when map works you’re just a masochist.

https://twitter.com/jaffathecake/status/1213077702300852224

Classic for-loops create state and ask you to manually manage control-flow using an arbitrary conditional expression. They open the door for a whole lot of subtle mistakes that for..of can protect you from in the cases where it applies (which in my experience, are 95% of the cases where you’d use a for loop at all)

I get where you are coming from, but it is obvious to me `forEach` is side effect because of its return type.

Also I think `forEach` is great for cases such as `arr.forEach(f)` where `arr: Array<T>, f: T => void`.

The airbnb style guide bans usage of forEach too for the same reasons

for is awkward in JS because you get either keys or values and not both

ForEach does get you both.

I don't remember using for loop for at least 3 years previously as a frontend engineer :)

for (const [index, value] of array.entries()) {}

It is longer than:

array.forEach((value, index) => {})

… but forEach has too many limitations (no exits, no await, …)

oh yeah, destructuring's nice

I'll give you await (gotta use awkward reduce there) but if you want exit, you're better off with something like `find`

Another advantage is that array functions better convey what are you trying to do with this particular operation.

So basically only Miranda and Haskell are functional.

Haskell has `forM_` (in Control.Monad), so no luck :(

That's still functional, monads aren't impure or anything :)

> IMO it's the worst of both worlds and there's virtually never a good reason to use it.

I strongly disagree.

map() is great in most languages but JS manages to WTF it:

  > ['1', '2', '3'].map(parseInt)
  [1, NaN, NaN]

  > ['3', '2', '1'].map(parseInt)
  [3, NaN, 1]

This problem isn’t about map(). parseInt takes a second argument - a radix - and map passes a second argument to to the callback: an index

The normal solution is to write an inline function; “shortcutting” it by just passing a named function directly can be nice when it’s appropriate, but it often isn’t what you want to do

Coming from outside the JS world, the problem is exactly about surprising behaviour of map - the function that's probably the oldest and best estabilished element of FP vocabulary. The extra index argument is not present in functions called "map" in other languages.

Also, the silently ignored argument count mismatch errors heavily contribute to the problem. You may have used other functions with map() and never knew it's always passing an extra argument in, because JS silently ignores this error.

Sorry if it’s unfamiliar, but the index argument is critical to many real use-cases.

As far as ignoring errors silently, sure, but I’m not even sure what the error would be in this case. Index is a number and radix is a number; even typescript won’t complain about this because the types line up.

Is it critical in some way that's absent from all the other langugaes that have a map()? There are standard, composable, explicit patterns in Python, Clojure, and I presume other map()-equipped anguages to get you the indices along with the items. (enumerate in Python and map-indexed in Clojure)

The error I was talking about was the argument count mismatch erorr, that would have taught you on all the other times you used map() without knowing about the extra index argument.

> Is it critical in some way that’s absent from all the other langugaes that have a map()?

Yes, but only because of other non-ergonomic design decisions of the API its part of (particuarly, that it is an eager API attached to the Array class returning Arrays; if it were a lazy API of generalized iterable protocol that returned and could be used on generators, then you could just also have an “enumerate” and “with_collection” companion methods, but as it is, even if you made standalone functions for those purposes, for the output to be used with .map() you need them to return Arrays, which means you are accumulating bulky intermediate results.)

Yes, eagerness would use additional space in the case of a Python enumerate() style solution. This would be acceptable in my view and aligned with the general JS design. But an alternative is the map-indexed style solution (aka the current Array.map with a different name).

> Sorry if it’s unfamiliar, but the index argument is critical to many real use-cases.

Only because .map() is an Array method, not a method of a generic iteration protocol applicable to other iterables (including generators.)

In Python, while map is a built-in function instead of a method, and you’ll probably want a comprehension instead, if you want the index for a map (or in a comprehension), you just wrap call enumerate() and go from an iterable of items to an iterable of pairs of (item, index), so:

  map(f, enumerate(collection))

In Ruby, if you want the index in a map, you just do:

   collection.each_with_index.map {|item, index| … }

I ran across this anecdote about how people don't know (or acknowledge?) the existence of the index argument of Array.map. This is a link to the manuscript of the well regarded book on JS FP techniques "Javascript Allonge". The book was linked elsewhere in these comments. The index arg is not mentioned!

https://github.com/raganwald/javascript-allonge/blob/db7c435...

> This problem isn’t about map().

Yes, it is.

Its about .map(), and the broader design of the API around it. Yes, map passes extra args and parseInt takes optional args, and it wouldn’t be a problem if both of those weren’t true. It would be better if .map() was one-arg and had friends .map_with_index(), .map_with_collection(), map_with_index_and_collection().

(It would be better if .map() wasn’t an Array-specific method but part of a general collection protocol that returned [and was applicable to] generators instead of returning Arrays, and then you also had companion methods to like .enumerate(), but that’s…well, farther afield.)

> The normal solution is to write an inline function

Yes, that’s the solution, but that doesn’t say that the design of .map() isn’t a critical component of there being something to solve in the first place.

I think it is important that you brought awareness to this, this is a really good point and I agree 100%. JavaScript does have hygiene issues with its API design.

From https://stackoverflow.com/questions/22036300/what-is-1-2-3-m...

// Consider: ["1", "2", "3"].map(parseInt); // While one could expect [1, 2, 3] // The actual result is [1, NaN, NaN]

// parseInt is often used with one argument, but takes two. The second being the radix // To the callback function, Array.prototype.map passes 3 arguments: // the element, the index, the array // The third argument is ignored by parseInt, but not the second one, // hence the possible confusion. See the blog post for more details

function returnInt(element){ return parseInt(element,10); }

["1", "2", "3"].map(returnInt); // Actual result is an array of numbers (as expected)

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