[swift-evolution] Tuple conversion and type composition

[Andrew Bennett]

Hi Swift Evolution populace,

I'd like a a way to concatenate tuple types together:

typealias ABCD = (A,B)+(C,D) // Same as (A,B,C,D)

Also a way to allow tuples to be converted to other tuples that are the
same when flattened:

(a,(b,c),d) as ((a,b),(c,d))

This proposal aims to make the minimal changes possible, with the most
familiar syntax, and give tuples this power.
Examples
These are things that will be possible after these changes.
Joining two tuples together

let a = (1,2), b = (3,4)let c = join(a, b) // (Int,Int,Int,Int)

Stack like operations on tuples

let (abcde)  = (1,2,3,4,5)let (abcd,e) = pop(abcde)let (abc,d)  =
pop(abcd)let (abce)   = push(abc, e)assert(abce == (1,2,3,5))

Chained ZipSequence

let a = [1,2,3]let b = ["a", "b", "c"]let c = [1.2, 2.3, 3.4]let d =
[nil, nil, 123]let abc = a.zip(b).zip(c).zip(d) //
AnySequence<(Int,String,Float,Int?)>

Turn any function with a callback into one with a promise

func promise<A,B,C>(f: (A + (B->Void)) -> C) -> A -> (C, Promise<B>)

This proposal, and more examples, can be seen in more detail here:

https://github.com/therealbnut/swift-evolution/blob/therealbnut-tuple-manipulation/proposals/0000-tuple-operators.md

I'll keep this file up to date over the course of the discussion, PRs are
welcome.

*Related proposals:*

This proposal's use cases relate to at least a few other currently active
proposals:

   - *Compile-time parameters*
   - *http://thread.gmane.org/gmane.comp.lang.swift.evolution/5240
      <http://thread.gmane.org/gmane.comp.lang.swift.evolution/5240>*
   - *Contiguous Variables (A.K.A. Fixed Sized Array Type)*
      -
*http://thread.gmane.org/gmane.comp.lang.swift.evolution/4809
      <http://thread.gmane.org/gmane.comp.lang.swift.evolution/4809> *
   -
*Remove implicit tuple splat behavior from function applications *
      - *http://thread.gmane.org/gmane.comp.lang.swift.evolution/4681
      <http://thread.gmane.org/gmane.comp.lang.swift.evolution/4681>*

Thanks!
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