[swift-evolution] Pitch: "while" clause on for-loops

[Chris Lattner]

> On Dec 31, 2015, at 12:42 AM, Jacob Bandes-Storch via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Currently, for-loops admit a "where" clause:
> 
>     for x in seq where cond {
>         ...
>     }
> 
>     behaves like
> 
>     for x in seq {
>         if !cond { continue }
>         ...
>     }
> 
> 
> I'd be interested in a "while" clause:
> 
>     for x in seq while cond {
>         ...
>     }
> 
>     would behave like
> 
>     for x in seq {
>         if !cond { break }
>         ...
>     }
> 
> 
> This is one area where C-style for-loops would have provided a clean solution (combining multiple conditions with &&), but once they're removed any extra conditions will have to move inside the loop.
> 
> Pros:
> - It's a simple way to express a common piece of control flow that otherwise requires negation and another set of braces.
> - Its meaning is easy to understand.
> 
> Cons:
> - It's a new feature.
> - Naming might cause confusion with while-loops.
> 
> Open questions:
> - How/could it be combined with "where" clauses? Would order matter?
> - Does anyone else care?

My 2c: it is adding complexity and potential for confusion for very little gain. Also, “where” and “while” are structurally different, because while isn’t a modifier that occurs in many other places.  Adding “where” to for loops was justified by a push to improve pattern matching in swift.

-Chris

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