Return value of forEach

Isiah Meadows isiahmeadows at gmail.com
Fri Oct 16 18:51:23 UTC 2015 

What about a `tap` function similar to Bluebird's `tap` function for its
promises?

I frequently write a `pipe` function that deals with this common case (with
arrays, but frequently promises and other monadic structures as well).

```js
// Complete transparent wrapper
function pipe(f) {
  function g(x) {
    f.apply(this, arguments)
    return x
  }

  return Object.defineProperty(g, "length",
    Object.getOwnPropertyDescriptor(f, "length"))
}

// My usual lazy version
function pipe(f) {
  return function (x) {
    f.apply(this, arguments)
    return x
  }
}

// My super-lazy version
const pipe = f => x => (f(x), x)
```

On Fri, Oct 16, 2015, 12:52 Kris Kowal <kris.kowal at cixar.com> wrote:

> forEach should return undefined. Array-like methods for iterators solve
> the problem of building unnecessary memory pressure for intermediate maps.
>
> The return value of forEach is special. True, for arrays it is always
> undefined. However, the forEach method on an iterator returned by a
> generator function should be the return value of the generator. Consider
> this range generator that returns the number of visited values.
>
> ```js
> function *range(start, stop, step) {
>     for (var index = start; index < stop; stop += step) {
>         yield index;
>     }
>     return (stop - start) / step;
> }
> var iterator = range(0, 10, 1);
> var iterations = iterator.forEach((x) => console.log(x));
> console.log(iterations);
> ```
>
> Kris Kowal
>
> On Fri, Oct 16, 2015 at 7:30 AM, Andrea Giammarchi <
> andrea.giammarchi at gmail.com> wrote:
>
>> That's usually a made-up issue ... the example code is using two maps
>> instead of computing the entire thing once in a composed function **and**
>> it already creates multiple copies of the initial Array.
>>
>> Anyway, having `var log = DEBUG ? x => console.log(x) || x : x => x` in
>> means you could do this:
>>
>> ```js
>> const a = log([1, 2, 3]
>>   .map(square))
>>   .map(plus1)
>>   .reduce(add);
>> ```
>>
>> but again, we are using multiple maps and a reduce so if we want to be
>> realistic, and it's for debugging purpose only, I think using map would be
>> just fine.
>>
>> Regards
>>
>>
>> On Fri, Oct 16, 2015 at 3:17 PM, Eric Devine <devineej at gmail.com> wrote:
>>
>>> Array#map does the non-trivial operation of copying the entire array. In
>>> the example for loging to the console, this behavior is unintended. Perhaps
>>> an underscore-like tap method should be considered to be added to the Array
>>> prototype for these cases.
>>>
>>>
>>> On Fri, Oct 16, 2015 at 9:32 AM, Andrea Giammarchi <
>>> andrea.giammarchi at gmail.com> wrote:
>>>
>>>> Like I've written before, if you need to return an array you can use
>>>> map instead of forEach
>>>>
>>>> `arr.map(x => console.log(x) || x)`
>>>>
>>>> forEach has been like this since ever so while you wonder what kind of
>>>> code would look like based on the fact forEach doesn't return  anything, I
>>>> actually wonder how come there are still people expecting forEach to return
>>>> something.
>>>>
>>>> So, it would be a breaking change and actually it's not needed since
>>>> you have map.
>>>>
>>>> If the new Array, and for debugging purpose, is not desired, use `var
>>>> log = DEBUG ? x => console.log(x) || x : x => x` and abuse it as much as
>>>> you like.
>>>>
>>>> Would any of this work?
>>>>
>>>> Regards
>>>>
>>>>
>>>>
>>>> On Fri, Oct 16, 2015 at 2:23 PM, Niloy Mondal <niloy.mondal84 at gmail.com
>>>> > wrote:
>>>>
>>>>> > That'd be a compatibility break.
>>>>>
>>>>> Ugh... you mean people actually depend on `forEach` returning
>>>>> `undefined` (which it always does) to do further task?
>>>>>
>>>>> I wonder what that kinda code would look like >.<
>>>>>
>>>>> On Fri, Oct 16, 2015 at 6:08 PM, Frankie Bagnardi <
>>>>> f.bagnardi at gmail.com> wrote:
>>>>>
>>>>>> That'd be a compatibility break.
>>>>>>
>>>>>> If we end up getting :: though:
>>>>>>
>>>>>> ```js
>>>>>> function logEach(){
>>>>>>   this.forEach((x) => console.log(x));
>>>>>>   return this;
>>>>>> }
>>>>>>
>>>>>>
>>>>>> const a = [1, 2, 3]
>>>>>>   .map(square)
>>>>>>   ::logEach()
>>>>>>   .map(plus1)
>>>>>>   .reduce(add);
>>>>>> ```
>>>>>>
>>>>>> You could make that a global variable so you can sprinkle it around
>>>>>> your code in development.
>>>>>>
>>>>>> Having some developer tools in the language would be nice though. I
>>>>>> don't even think console.log is in the spec. A global like Debug.logThis
>>>>>> for example would be a more general ::-able.
>>>>>>
>>>>>>
>>>>>> On Fri, Oct 16, 2015 at 5:32 AM, Andrea Giammarchi <
>>>>>> andrea.giammarchi at gmail.com> wrote:
>>>>>>
>>>>>>> ```js
>>>>>>> const a = [1, 2, 3]
>>>>>>>   .map(square)
>>>>>>>   .map(x => console.log(x) || x )
>>>>>>>   .map(plus1)
>>>>>>>   .reduce(add);
>>>>>>> ```
>>>>>>>
>>>>>>> Regards
>>>>>>>
>>>>>>>
>>>>>>> On Fri, Oct 16, 2015 at 10:13 AM, Niloy Mondal <
>>>>>>> niloy.mondal84 at gmail.com> wrote:
>>>>>>>
>>>>>>>> Currently, `Array.prototype.forEach` returns `undefined`. It would
>>>>>>>> be more
>>>>>>>> useful if it returns the array itself.
>>>>>>>>
>>>>>>>> Say I have written some code like this...
>>>>>>>>
>>>>>>>> ```js
>>>>>>>> const a = [1, 2, 3]
>>>>>>>>   .map(square)
>>>>>>>>   .map(plus1)
>>>>>>>>   .reduce(add);
>>>>>>>> ```
>>>>>>>>
>>>>>>>> For some reason, I am not getting the expected output. For
>>>>>>>> debugging, I would
>>>>>>>> like the print the values after each step. My first initial
>>>>>>>> reaction is to
>>>>>>>> put a `forEach` step in between and print the values like so...
>>>>>>>>
>>>>>>>> ```js
>>>>>>>> const a = [1, 2, 3]
>>>>>>>>   .map(square)
>>>>>>>>   .forEach(x => console.log(x))
>>>>>>>>   .map(plus1)
>>>>>>>>   .reduce(add);
>>>>>>>> ```
>>>>>>>>
>>>>>>>> Unfortunately, this does not work as `forEach` returns `undefined`.
>>>>>>>> I now have
>>>>>>>> to comment out all the code below it. Having the _plug and play_
>>>>>>>> behaviour for
>>>>>>>> `forEach` would be very convenient.
>>>>>>>>
>>>>>>>> _______________________________________________
>>>>>>>> es-discuss mailing list
>>>>>>>> es-discuss at mozilla.org
>>>>>>>> https://mail.mozilla.org/listinfo/es-discuss
>>>>>>>>
>>>>>>>>
>>>>>>>
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>>>>>>>
>>>>>>
>>>>>
>>>>
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>>>
>>
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