# V2041 assign_captured_var

URL: /docs/errors/V2041
Markdown: /docs/errors/V2041.md

> A lambda, or a try body, assigns to a var declared outside it.

An error from the `type` phase. `sakura explain V2041` prints this explanation offline.

A lambda, or a `try` body, assigns to a `var` declared outside it. Closures capture `var`s by
value, so the assignment would be lost (design §8.1); a `try` body's only outputs are its value
and its error (design §5.3). Use a loop with the `var` in the same function, or return the new
value and assign it afterwards.

Wrong:

```sakura
var n = 0
let r = try { n = parse(s)? }
```

Fixed:

```sakura
var n = 0
let r = try { parse(s)? }
n = r.unwrap_or(0)
```

## Wrong program [#wrong-program]

[`compiler/tests/ui/V2041_assign_captured_var.sakura`](https://github.com/icantcodefyi/sakuralang/blob/master/compiler/tests/ui/V2041_assign_captured_var.sakura), which the compiler's ui suite runs through `sakura check`:

```sakura
fn total() -> Int {
  var sum = 0
  let add = fn(x) { sum = sum + x }
  add(1)
  sum
}
```

`sakura check --json` reports this record (pretty-printed; `$DIR` is the fixture's directory):

```json
{
  "schema_version": "1.0",
  "kind": "diagnostic",
  "id": "d:c2398541ac6c660a",
  "code": "V2041",
  "severity": "error",
  "phase": "type",
  "message": "this function value assigns `sum`, a `var` of the enclosing function; closures capture variables by value, so the assignment would be lost",
  "snapshot": "sha256:$SNAPSHOT",
  "symbol_id": "main::V2041_assign_captured_var::total",
  "spans": [
    {
      "uri": "file://$DIR/V2041_assign_captured_var.sakura",
      "content_hash": "sha256:97a19a1223e83309a662c4808f5fc8387a02dbebe4f7ad50d975c397cf561e6d",
      "start_byte": 88,
      "end_byte": 101,
      "role": "primary"
    }
  ],
  "related_ids": [],
  "expected": null,
  "actual": null,
  "obligation": null,
  "counterexample": null,
  "explanation_uri": "sakura://explain/V2041",
  "fixes": []
}
```

More fixtures for this code: [`compiler/tests/ui/V2041_assign_in_try.sakura`](https://github.com/icantcodefyi/sakuralang/blob/master/compiler/tests/ui/V2041_assign_in_try.sakura).

## Fixed program [#fixed-program]

[`compiler/tests/ui/V2041_assign_captured_var.fixed.sakura`](https://github.com/icantcodefyi/sakuralang/blob/master/compiler/tests/ui/V2041_assign_captured_var.fixed.sakura), which `sakura check` accepts with no diagnostics:

```sakura
fn total() -> Int {
  var sum = 0
  for x in [1] { sum = sum + x }
  sum
}
```