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GALA-E0115 — an inout place used inside Try

When it fires. The thunk of Try(...) (a lambda, or an expression passed by name) reads or writes an inout place. Try catches a panic, so a write made before the panic would stay in the caller’s place while the call reports a failure: the caller would see a half-updated value.

Minimal repro.

package main

struct Counter(var n int)

func step(inout c Counter) = Println(Try(() => {
    c.n = c.n + 1
    c.n
}))

func main() {
    var c = Counter(1)
    step(&c)
    Println(c.n)
}

Error output.

error[GALA-E0115]: inout place c is used inside Try: a panic Try catches would leave the caller's place half updated
  --> main.gala:6:5
  |
6 |     c.n = c.n + 1
  |     ^ copy it into a local `var` first, and assign c back once Try…
  |
  = hint: copy it into a local `var` first, and assign c back once Try has succeeded

Fix. Work on a copy inside Try, and write the place once Try has succeeded:

package main

struct Counter(var n int)

func step(inout c Counter) {
    val start = c.n
    Try(() => start + 1) match {
        case Success(n) => c.n = n
        case Failure(_) => Println("unchanged")
    }
}

func main() {
    var c = Counter(1)
    step(&c)
    Println(c.n) // 2
}

Rationale. A caller sees an inout place change all at once, when the call returns. Ruling out writes inside a region that catches panics keeps that true, and keeps the translation of the function to value semantics, which returns the new value with the result, faithful.