[swift-evolution] Extending the for loop to have multiple clauses

[thorsten at portableinnovations.de]

Having a notation instead of just a function enables lots of sweet stuff not just nested loops (which happens within the List monad) but more, depending on the monad being used. Just have a look at Scala's for-expression, Haskell's do-notation or C#'s LINQ, e.g.

let result: Future<Int> = for
    x in getXAsynchronously(),
    y in getYAsynchronously() {
        x + y
    }

using an Async Monad here, where flatMap() is defined appropriately.

-Thorsten

> Am 09.12.2015 um 23:38 schrieb Colin Barrett via swift-evolution <swift-evolution at swift.org>:
> 
> Indeed. In fact, ilya posted this already with an implementation upthread :) 
> 
> BTW, the mathematical term is “Cartesian product”, named so because this operation is how you construct the famous Cartesian xy-coordinate system we all learned in Algebra class.
> 
> FWIW I agree that this should be a library function and not in the language.
> 
> -Colin
> 
>> On Dec 9, 2015, at 5:31 PM, David Waite via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> You could have a function (lets call it cross for lack of me knowing a better mathematical term) that takes in multiple CollectionTypes and returns a CollectionType of all the combinations of the constituent sequence type elements.
>> 
>> for (x,y) in cross(0..<width, 0..<height) { … }
>> 
>> My swift-fu however is not yet strong enough for me to know how to avoid the backing generators being CrossGenerator2, CrossGenerator3, etc.
>> 
>> -DW
>> 
>>> On Dec 9, 2015, at 3:11 PM, Chris Eidhof via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> Exactly! Just like multiple-if-let is the same as flatMap on optionals, this is a better syntax for flatMap on arrays. 
>>> 
>>> To make it more clear that it’s a nested loop, we could also consider:
>>> 
>>> for x in xs, for y in ys {
>>> }
>>> 
>>> But I’m not sure if it actually is clearer. (You could totally still interpret this as a zip, rather than a flatMap).
>>> 
>>> Chris
>>> 
>>> 
>>> 
>>>>> On 09 Dec 2015, at 16:51, Maxwell Swadling <maxs at apple.com> wrote:
>>>>> 
>>>>> 
>>>>>> On 9 Dec 2015, at 1:44 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org> wrote:
>>>>>> 
>>>>>> 
>>>>>> On Dec 9, 2015, at 1:43 PM, Brent Royal-Gordon <brent at architechies.com> wrote:
>>>>>> 
>>>>>>>> I think it could be really nice to extend the for-loop so that it can have multiple clauses. Much like in the if-let with multiple clauses, I could imagine a for-loop with multiple clauses:
>>>>>>>> 
>>>>>>>> var cards: [(Suit,Rank)] = []
>>>>>>>> for x in suits, y in ranks {
>>>>>>>> cards.append((x,y))
>>>>>>>> }
>>>>>>> 
>>>>>>> 
>>>>>>> Isn’t this just
>>>>>>> 
>>>>>>> 	for (x, y) in zip2(suits, ranks) {
>>>>>>> 	}
>>>>>>> 
>>>>>>> ?
>>>>>> 
>>>>>> No, apparently it’s supposed to be all permutations of both types. But I too read it as syntactic sugar for a zip, which I think is a great reason not to add this syntax.
>>>>> 
>>>>> 
>>>>> Ah. In that case, I’d much rather have a library function that indicates that we’re getting all permutations. This doesn’t feel like it belongs in the language at all, but in the library.
>>>>> 
>>>>> 	- Doug
>>>>> 
>>>>> _______________________________________________
>>>>> swift-evolution mailing list
>>>>> swift-evolution at swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>> 
>>>> 
>>>> We do have this library function, it is flatMap.
>>>> 
>>>> The desired feature here seems to be desugaring for loops  
>>>> 
>>>> for x in xs, y in ys, z in zs {
>>>>   ...
>>>> }
>>>> 
>>>> into:
>>>> 
>>>> xs.flatMap { x in ys.flatMap { y in zs.map { z in ... } } }
>>>> 
>>>> So as above you would get:
>>>> 
>>>> suits.flatMap { x in ranks.map { y in cards.append((x,y)) } }
>>> 
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