E0136

Alternatives bind different names

Error

Built from the repository at commit a8614dc.

Each alternative of a | pattern binds the same names, with the same types, so the arm can use them whichever alternative matched.

Why

The arm's body runs after any one of the alternatives matches. A name that only some of them bind would have no value when another one matched. Bind the name in every alternative, or use _ in all of them.

Broken

enum Shape {
    Square(i64)
    Rect(i64, i64)
}

fn width(s: Shape) -> i64 {
    match s {
        Square(w) | Rect(_, h) => w
    }
}

fn main() {
    print(width(Shape.Square(2)))
    print(width(Shape.Rect(3, 4)))
}

What sisu says

error[E0136]: `w` is bound in one alternative but not the other
 --> E0136.sisu:8:21
  |
8 |         Square(w) | Rect(_, h) => w
  |                     ^^^^^^^^^^

sisu: 1 error
sisu: for more on a code, run `sisu explain <code>`: E0136

Fixed

enum Shape {
    Square(i64)
    Rect(i64, i64)
}

fn width(s: Shape) -> i64 {
    match s {
        Square(w) | Rect(w, _) => w
    }
}

fn main() {
    print(width(Shape.Square(2)))
    print(width(Shape.Rect(3, 4)))
}