[swift-evolution] [Proposal] Invert the order of pattern match operator

[Dave Abrahams]

on Thu Apr 07 2016, David Owens II <swift-evolution at swift.org> wrote:

> It would be nice to know the rationale behind the choice of the current syntax.
> I agree that these seem more natural:
>
>     @warn_unused_result
>     public func ~=<I : ForwardIndexType where I : Comparable>(value: I, pattern:
>     Range<I>) -> Bool
>
>     @warn_unused_result
>     public func ~=<I : IntervalType>(value: I.Bound, pattern: I) -> Bool

+1

> I would not change from `~=` to `=~` though.

One downside with `~=` is that it reads like `+=`, which mutates the
LHS.  Of course, `=~` has its own issues, e.g. `x=~y` might read as `x =
~y`.

> So you have this:
>
>     let x = 4
>
>     switch x {
>     case let v where x ~= 0...5: print("matched: \(v)")
>     default: print("nothing!!")
>     }
>
> -David
>
>     On Apr 7, 2016, at 4:57 AM, David Rodrigues via swift-evolution
>     <swift-evolution at swift.org> wrote:
>
>     Hi all,
>
>     Swift has a pattern match operator, ~=, which is unknown to many (like me
>     until a few weeks ago), that performs a match between a value and a certain
>     pattern, e.g. checking if an integer value is contained in a range of
>     integers.
>
>     This operator may be little known, but it plays a key role in the language
>     since it's used behind the scenes to support expression patterns in `switch
>     ` statement case labels, which we all know are extremely popular.
>
>     let point = (2, 4)
>     switch point {
>     case (0, 0):
>     print("The point is at the origin")
>     case (0...4, 0...4):
>     print("The point is in the subregion")
>     default:
>     break
>     }
>
>     Most of the time we don't use the operator directly but it is available and
>     can be handy in certain conditions.
>
>     let point = (2, 4)
>     switch point {
>     case (let x, let y) where 0...4 ~= x && 0...4 ~= y:
>     print("The point is in the subregion")
>     default:
>     break
>     }
>
>     However the current syntax is not ideal (in my opinion). We're not really
>     declaring the operation that we want to do, and that has an impact in the
>     expressivity and readability of the code. Currently we're doing matches like
>     "if blue is the ocean" instead of "if the ocean is blue" or "if the ocean
>     contains the whale" instead of "if the whale is in the ocean".
>
>     For that reason, I would like to suggest inverting the order of the operator
>     to match more closely our logical thought.
>
>     case (let x, let y) where x =~ 0...4 && y =~ 0...4: // Proposed
>     // vs
>     case (let x, let y) where 0...4 ~= x && 0...4 ~= y: // Current
>
>     I have an ongoing proposal to suggest this change and it contains a little
>     more context. It is available here: 
>
>     https://github.com/dmcrodrigues/swift-evolution/blob/proposal/invert-order-of-pattern-match-operator/proposals/NNNN-invert-order-of-pattern-match-operator.md
>    .
>
>     Any feedback is very welcome.
>
>     Thank you.
>
>     David Rodrigues
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-- 
Dave