E0039

Parameter name used twice

Error

Built from the repository at commit a8614dc.

Each parameter of a function or method needs a name of its own. A let or let mut may reuse a name, a parameter's included: the new binding shadows the old one from its declaration onwards.

Why

A call passes its arguments by position, but the body reads them by name. With two parameters named a, a read of a could mean either argument.

Broken

fn add(a: i64, a: i64) -> i64 {
    a + a
}

fn main() {
    print(add(1, 2))
}

What sisu says

error[E0039]: `a` is already declared in this scope
 --> E0039.sisu:1:16
  |
1 | fn add(a: i64, a: i64) -> i64 {
  |        -       ^
  |        |
  |        first declared here

sisu: 1 error
sisu: for more on a code, run `sisu explain <code>`: E0039

Fixed

fn add(a: i64, b: i64) -> i64 {
    a + b
}

fn main() {
    print(add(1, 2))
}