[swift-evolution] [proposal] Allow trailing closures in 'guard' conditions

[Chris Lattner]

Hi everyone,

This is a proposal for a very narrow extension to the guard statement.  I consider it to be a bug fix, but since it is a language extension, I feel that it should go through the evolution process.  Thoughts appreciated!

-Chris


Introduction & Motivation

The three statements `if`, `while`, and `guard` form a family that all take a rich form of conditions that can include one or more boolean conditions, `#available` clauses, and `let`/`case` pattern bindings.  These are described by the `condition-clause` production in the TSPL reference section and as a `stmt-condition` in the compiler source code.

Today, these do not permit trailing closures in any top-level expressions embedded in the condition, because that would be generally ambiguous with the body of an if or while statement:

	if foo {   // start of trailing closure, or start of the if body?

While it would be possible to tell what is intended in some cases by performing arbitrary lookahead or by performing type checking while parsing, these approaches have significant consequences for the architecture for the compiler.  As such, we’ve opted keep the parser simple and disallow this.  Unrelated to this proposal, I landed a patch (https://github.com/apple/swift/commit/30ec0f4128525a16f998e04ae8b1f70180627446 <https://github.com/apple/swift/commit/30ec0f4128525a16f998e04ae8b1f70180627446>) which *greatly* improves the error messages in some of the most common cases where a developer accidentally tries to do this. 

However, while this approach makes sense for `if` and `while` statements, it does not make sense for ‘guard': The body of a guard statement is delineated by the `else` keyword, so there is no ambiguity.  A brace is always the start of a trailing closure.

From a historical perspective, the current situation was an oversight.  An earlier design for `guard` did not include the `else` keyword (it used the `unless` keyword), and I forgot to fix this when we decided to resyntax it to `guard/else`. 


Proposed solution

The solution is simple: allow trailing closures in guard bodies.  This would allow this silly example to compile correctly:

func f(arr : [Int]?) {
  guard let x = arr?.map {$0+1} else {
    preconditionFailure()
  }

  // ...
}

Detailed Design

The impact on the compiler is trivial, here’s a patch:




Impact on existing code

There is no impact on existing code.  This only makes formerly invalid code start being accepted.


Alternatives considered

There are three primary alternatives: do nothing, expand the scope of ‘if' and ‘while’ conditions as well, and significantly change the syntax of guard.

Do nothing: It can be argued that this change would make guard inconsistent with the restrictions of ‘if’ and ‘while’ and that inconsistency would be confusing.  On the other hand, I am arguing that this is an arbitrary restriction.

Expand the scope of “if” and “while” statements:  Through enough heroics and lookahead we could consider relaxing the trailing closure requirements on `if` and `while` statements as well.  While this could be interesting, it raises several ambiguity questions.  For example, we need significant lookahead to realize that “a” here is not a trailing closure, since we have a closure expression being fully applied after it:

	if foo { …a... } { …b… }()

this could be addressed with whitespace rules or other approaches, but since any such direction would be compatible with this proposal, I see it as a separable potential extension on top of this basic proposal.

Change the syntax of guard: I only list this for completeness, but we could eliminate the `else` keyword, making guard more similar to `if` and `while`.  I personally think that this is a really bad idea though: the guard statement is not a general `unless` statement, and its current syntax was very very carefully evaluated, iterated on, discussed, and re-evaluated in the Swift 2 timeframe.  I feel that it has stood the test of time well since then.


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