# XO0532: a Map value read, changed, and stored back

Reported by `xo vet`, not `xo check`. `var x = m.get(k) ?? d`, statements that only change x, then `m[k] = x` looks the key up twice and depends on the last line: values are copies (core 2.6), so without it the change is lost (the checker's XO0520). `m.update(k, d, fn(var x) { ... })` reads k (or d), runs the function on the value in place, stores it, and returns the function's value (std.md 2.4). The fix rewrites the statements into that call. See decision 0266.

## Example

```xo
fn index(var by: Map[Str, List[Int]], name: Str, i: Int) {
  var ids = by.get(name) ?? []
  ids.push(i)
  by[name] = ids
}
```

## Fix

```xo
fn index(var by: Map[Str, List[Int]], name: Str, i: Int) {
  by.update(name, [], fn(var ids) {
    ids.push(i)
  })
}
```

Run `xo explain XO0532` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).