Proposal to remove C-Style For-Loops from Swift 3.0

erica-proposal.md

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+# Remove C-style for-loops with conditions and incrementers
+
+* Proposal: [SE-NNNN](https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md)
+* Author(s): [Erica Sadun](https://github.com/erica)
+* Status: **Review**
+* Review manager: TBD
+
+## Introduction
+
+The C-style `for-loop` appears to be a mechanical carry-over from C rather than a
+genuinely Swift-specific construct. It is rarely used and not very Swift-like. 
+
+More Swift-typical construction is already available with `for-in`
+statements and `stride`. Removing for loops would simplify the language and starve the
+most common use-points for `--` and `++`, which are already due to be eliminated from the
+language.
+
+The value of this construct is limited and I believe its removal should be seriously considered.
+
+
+## Advantages of For Loops
+
+Swift design supported a shallow learning curve using familiar constants and control
+structures. The `for-loop` mimics C and limits the effort needed to master this control flow.
+
+## Disadvantages of For Loops
+
+1. Both `for-in` and `stride` provide equivalent behavior using Swift-coherent approaches 
+   without being tied to legacy terminology. 
+1. There is a distinct expressive disadvantage in using `for-loops` compared to `for-in` 
+   in succinctness
+1. `for-loop` implementations do not lend themselves to use with collections and other core Swift types.
+1. The `for-loop` encourages use of unary incrementors and decrementors, which will be
+   soon removed from the language.
+1. The semi-colon delimited declaration offers a steep learning curve from users arriving
+   from non C-like languages
+1. If the `for-loop` did not exist, I doubt it would be considered for inclusion in Swift 3.
+
+## Proposed Approach
+
+I suggest that the for-loop be deprecated in Swift 2.x and removed entirely in Swift 3, with coverage removed from the Swift Programming Language to match the revisions in the current 2.2 update.
+
+## Alternatives considered
+
+Not removing `for-loop` from Swift, losing the opportunity to streamline the language
+and discard an unneeded control flow item.
+
+## Impact on existing code
+
+A search of the Apple Swift codebase suggests this feature is rarely used. Community members of the Swift-Evolution mail list confirm that it does not feature in many pro-level apps and can be worked around for those few times when `for-loop`s do pop up. For example:
+
+```swift
+char *blk_xor(char *dst, const char *src, size_t len)
+{
+ const char *sp = src;
+ for (char *dp = dst; sp - src < len; sp++, dp++)
+   *dp ^= *sp;
+ return dst;
+}
+```
+
+versus
+
+
+```swift
+func blk_xor(dst: UnsafeMutablePointer<CChar>, src:
+UnsafePointer<CChar>, len: Int) -> UnsafeMutablePointer<CChar> {
+   for i in 0..<len {
+       dst[i] ^= src[i]
+   }
+   return dst
+}
+```
+
+A search of github's Swift gists suggests the approach is used primarily by those new to the language with minimal language skills and is abandoned as language mastery is achieved.
+
+For example:
+
+```swift
+for var i = 0 ; i < 10 ; i++ {
+    print(i)
+}
+```
+
+and 
+
+```swift
+var array = [10,20,30,40,50]
+for(var i=0 ; i < array.count ;i++){
+    println("array[i] \(array[i])")
+}
+```
+
+## Community Responses
+* "I am certainly open to considering dropping the C-style for loop.  IMO, it is a rarely used feature of Swift that doesn’t carry its weight.  Many of the reasons to remove them align with the rationale for removing -- and ++. " -- Chris Lattner, clattner@apple.com
+* "My intuition *completely* agrees that Swift no longer needs C-style for loops. We have richer, better-structured looping and functional algorithms. That said, one bit of data I’d like to see is how often C-style for loops are actually used in Swift. It’s something a quick crawl through Swift sources on GitHub could establish. If the feature feels anachronistic and is rarely used, it’s a good candidate for removal." -- Douglas Gregnor, dgregor@apple.com
+* "Every time I’ve used a C-style for loop in Swift it was because I forgot that .indices existed. If it’s removed, a fixme pointing that direction might be useful." -- David Smith, david_smith@apple.com
+* "For what it's worth we don't have a single C style for loop in the Lyft codebase." -- Keith Smiley, keithbsmiley@gmail.com
+* "Just checked; ditto Khan Academy." -- Andy Matsuchak, andy@andymatuschak.org
+* "We’ve developed a number of Swift apps for various clients over the past year and have not needed C style for loops either." -- Eric Chamberlain, eric.chamberlain@arctouch.com
+* "Every time I've tried to use a C-style for loop, I've ended up switching to a while loop because my iteration variable ended up having the wrong type (e.g. having an optional type when the value must be non-optional for the body to execute). The Postmates codebase contains no instances of C-style for loops in Swift." -- Kevin Ballard, kevin@sb.org
+* "I found a couple of cases of them in my codebase, but they were trivially transformed into “proper” Swift-style for loops that look better anyway. If it were a vote, I’d vote for eliminating C-style." -- Sean Heber, sean@fifthace.com
+