map(Traversable, callable) doesn't return a Traversable?

[morrisonlevi]

I'm curious why map() when called with a traversable doesn't return a traversable? Seems odd. What's the design decision there? Simplicity?

[lstrojny]

Reason is, that depending on the Traversable implementation passed to map() I don't necessary know how to create the same Traversable (obviously). The reasons for "always return an array" are simplicity and consistency.

[lstrojny]

That said, if you have a good idea how to change that behavior, I would be very interested.

[morrisonlevi]

Is there a reason you would have to return the same kind of Traversable?

[lstrojny]

That’s true, could be anything like an ArrayIterator. But that would only add runtime costs so I am not sure how useful it would be. Also given that the PHP APIs like array_keys() and so on take, well, arrays, also returning arrays feels like the lowest common denominator.

[morrisonlevi]

Here is a simple example that illustrates why returning an iterator performs better for some uses; admittedly I am not sure if the example is perfectly compatible with this repo but you should understand it:

$data = new ArrayIterator(range(0, 1000)); //realistically obtained from somewhere else.
limit(map($data, function($value) { return 2 * value;}), 4);

If you return a lazy iterator you only have to map 4 values; if you return an array you have to map all 1000 values and then limit the top 4.

To be clear, I'm not arguing that you should change, I was mostly curious why you didn't take this approach as most functional languages use lazy evaluation for their utility functions like this.

[lstrojny]

Very good example and I agree there are cases where it would work better. What Functional PHP could offer is set of functions under a different namespace that always returns iterators. Let me think a little more about it, any input is of course appreciated and thanks for the discussion so far.

[igorw]

FYI there is also nikic/iter that always returns lazy iterators.

[lstrojny]

Yep, a very nice library.

[lstrojny]

Closing issue for now.