Open Radar

 
Summary:
Sometimes when pattern matching a case C you then also need to perform some test on the matched value x. If that test takes the form f(x) != nil then you may also want to bind the result of f(x), and currently that means you have to either call f twice or rewrite your case to use an if statement after destructuring.

That is, currently you have to write this:

switch v {
case let C(x):
    if let y = f(x) { return y }
    else { return nil } 
default:
    return nil
}

This can complicate control flow substantially since you may need to explicitly handle the non-binding case instead of just letting a default case catch anything failing the test.


I propose allowing where clauses to bind constants, clarifying and simplifying control flow:

switch v {
case let C(x) where let y = f(x):
    return y
default:
    return nil
}


Steps to Reproduce:
N/A

Expected Results:
N/A

Actual Results:
N/A

Regression:
N/A

Notes:
N/A