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

Thu Dec 10 10:32:32 CST 2015

I’ve thought a bit more about this, and now have changed my mind: I don’t think supporting multiple clauses in a for loop is a good idea anymore.

It’s confusing: people who’re not familiar with this concept from other languages expect a zip rather than a flatMap. I think the added value of having multiple clauses doesn’t really pay for this. 

I still think there’s a lot of value in having something like do-notation, but I also feel that that’s too big a step currently.

Chris

> On 10 Dec 2015, at 07:56, thorsten at portableinnovations.de wrote:
> 
> 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 <mailto: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 <mailto: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 <mailto: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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