[swift-evolution] Pitch: Guard for declaration

[Angelo Villegas]

Hi Everyone,

Back in the ternary discussion, I kinda suggested the use of Guard but then I realised it’s not a replacement for ternary nor an alternative, it should not be, but may be a helpful addition to Swift.

let a | x > 0 && x < 10 = “less than ten”
      | x > 10 && < 100 = “less than a hundred”
      | x > 100 = “more than a hundred”
      | otherwise = “probably a negative”

There’s someone who said that it’s reasonable in declarations and another said that he like to explore using the bar (or pipe) character (|).

The where clause should work well with declarations. For instance, consider a function which computes the number of solutions for a quadratic equation, ax^2 + bx + c = 0

let num | disc > 0  = 2
        | disc == 0 = 1
        | otherwise = 0
        where
            disc = a2 + b + c
            where
                a = 2 * 1
                b = 2 + 3
                c = 6 / 2

I believe it may also help lessen the number of lines (in many cases) from using if-else-if and/or switch statements.

Anyway, I just thought to start a pitch. If people will like the idea we might give a proper proposal for the boolean guard.

- Angelo

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