[mrcartmenez]

It’s also because it’s good practice to use the lowest power loop possible.

A for loop can implement a reduce, a reduce can implement a map, a map can implement a forEach, but not the inverse.

[thaumasiotes]

Huh? I don't follow.

  In [1]: def lred(function, iterable, initializer): 
     ...:   acc = initializer 
     ...:   def reductor(el): 
     ...:     nonlocal acc 
     ...:     acc = function(acc, el) 
     ...:   for n in map(reductor, iterable): pass 
     ...:   return acc 
     ...:                                                                             
  
  In [2]: lred(lambda a,b: 10*a + b, [2,4,6,3,1], 40)                                 
  Out[2]: 4024631

What's stopping you from running these equivalencies in either direction?

[paisawalla]

They mean that the forEach runs code per element, but cannot return a transformed list of values, you need map for that.

A map can run code per list element but the result must be injective (one-to-one).

Reduce can do all of the above, but must return a lower dimension result from its input.

And finally, a for loop is basically omnipotent.

[thaumasiotes]

A forEach can easily return a transformed list of values.

  acc = []
  for el in values:
    acc.append(f(el))
  return acc

This hierarchy doesn't exist; you can pretty much implement any of them in terms of the others.

To make that untrue, you need to define very strict limits on what else your language can do.

(Sure, we produce a transformed list here through the use of side effects. But note that the original example sparking the claim that forEach can't do what map can was this:

  .forEach(() => button.click())

Side effects are clearly allowed.)

[joe-user]

The forEach isn't returning it in your example, the line after it is returning. Try again without acc and I believe that's what was meant.

You can implement any of them in terms of the others, but only if you break convention and introduce side effects. Follow convention and try to implement (for example) reduce with map and you'll find that it's not possible.

[thaumasiotes]

> You can implement any of them in terms of the others, but only if you break convention and introduce side effects.

This can't be right. forEach has no effects other than side effects. A convention that says not to use side effects prevents you from using forEach at all.

But that would make claims about the place of forEach in a hierarchy into meaningless nonsense.

> The forEach isn't returning it in your example, the line after it is returning.

That depends on your point of view. C has numerous functions which accept pointer parameters and return values in those parameters. That's just the normal way to return multiple values in C.

And by that standard, the forEach itself is returning the transformed list; that's where the transformation occurs. The following `return` line is only necessary if this is a snippet within a function whose purpose is to execute a for statement; you would actually do this inline, by just writing the for statement without needing a following `return`.

[HeavyStorm]

I really don't think you got the original point...

[momentoftop]

Haskell has strict limits on side-effects, but also abstracts these functions over the thing you're "forEaching." There, a reduce can't implement a map, because a map is required to return the same type of collection you put in. A reduce can only return an arbitrarily chosen collection (say, an Array).

And without side-effects, a map can't implement a forEach.

[thaumasiotes]

> And without side-effects, a map can't implement a forEach.

Yes, it can, because without side effects, forEach is a NOP. That's not difficult to implement. As long as you don't do anything with the return value from map, you're there.

[seedboot]

forEach doesn't return a value https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

[DangitBobby]

> Reduce can do all of the above, but must return a lower dimension result from its input.

What do you mean by this? You can accumulate just about anything into the resulting object, including a copy of the original array.

[SamBam]

I think it's just being confusingly stated. GP already said that reduce could implement map. I think it's merely that reduce can just output a single result, of any type. (Which is already more powerful than map, which must output an array.)

Of course, in fact you could hack anything in any of these, since you could be doing other work in the function called. But I think the general principle is sound.

[thaumasiotes]

> (Which is already more powerful than map, which must output an array.)

Don't tell Common Lisp.

[jolux]

forEach is actually more powerful than map, you would not be able to use map for side effects in a static and/or lazy language. In a strict, dynamic language, it matters less. Even so I wouldn't describe being an expression vs statement as a difference in power. It's contextual.

[thaumasiotes]

> forEach is actually more powerful than map, you would not be able to use map for side effects in a static and/or lazy language.

You might notice this problem above, where it's necessary to do

  for n in map(reductor, iterable): pass

to realize the mapping.

[eru]

Especially, a for-loop can break out early or skip elements.

[thaumasiotes]

So can reduce. You'd put that logic in the function you pass to the call to reduce.

So can map. Ditto.

So can foreach. Ditto again.

  for el in values:
    if el == 3: pass
    else:
      do_something_with( el )

[mekkkkkk]

No, that's just a guard condition. Parent post was referring to break, i.e. skip the rest of the iteration.

[thaumasiotes]

For forEach that's easy:

  for el in values:
    if el == 3: return
    do_something_with( el )

For the other two, you still can, but you'll need to handle the control flow yourself, by doing something like goto or invoking a continuation.