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

[David Waite]

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 <mailto:maxs at apple.com>> wrote:
>> 
>> 
>>> On 9 Dec 2015, at 1:44 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>>> 
>>>> On Dec 9, 2015, at 1:43 PM, Brent Royal-Gordon <brent at architechies.com <mailto: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
>>> 
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>> 
>> 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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