[swift-evolution] proposal to add "in range" operator

[Gmail 2]

Hello everyone, 
I have a proposal to add an "in range” operator in swift.

##Current situation

Checking whether a value is or not within a certain range now depends on the control flow you use. for switch-statement, it is intuitive; but it is not  for if-statement.

Example:

let testValue = 0.3

switch testValue {
case 0.0..<1.0: do something...
default: break
}

to write the same logic in if-statement, you have to write like this:

if testValue >= 0.0 && testValue < 1.0 {
    do something...
}

The disadvantage here:
1. It’s wordy;
2. It’s risky to write the same variable name twice, typo can occur and be ignored;
3. It’s inconsistent with switch-statement.

##Proposed solution

Introduce a new operator to check a variable against a range.
Here is a drafted operator: ~ (infix operator)
With ~, the above if-statement can be rewritten like:

if testValue ~ 0.0..<1.0 {
    do something...
}

looks much better.

What’s more, if combined with the draft proposal Generalized range operators in `for-in`loops and other purposes <https://github.com/apple/swift-evolution/pull/129>, this new operator would more powerful.

Thanks for reading
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