E0129

Name bound twice in one pattern

Error

Built from the repository at commit a8614dc.

Each name in a pattern binds one field, so a name appears at most once in a pattern.

Why

In some languages Add(x, x) matches only when both fields are equal; in Sisu a pattern never compares two fields. Bind two names and compare them in a guard: Add(x, y) if x == y.

Broken

enum Expr {
    Add(i64, i64)
}

fn same(e: Expr) -> bool {
    match e {
        Add(x, x) => true
    }
}

fn main() {
    print(same(Expr.Add(1, 1)))
}

What sisu says

error[E0129]: `x` is bound twice in this pattern
 --> E0129.sisu:7:16
  |
7 |         Add(x, x) => true
  |             -  ^
  |             |
  |             first bound here
  |
  = help: use a guard: `Add(x, y) if x == y`

sisu: 1 error
sisu: for more on a code, run `sisu explain <code>`: E0129

Fixed

enum Expr {
    Add(i64, i64)
}

fn same(e: Expr) -> bool {
    match e {
        Add(x, y) if x == y => true
        _ => false
    }
}

fn main() {
    print(same(Expr.Add(1, 1)))
}