# V1040 assign_immutable

URL: /docs/errors/V1040
Markdown: /docs/errors/V1040.md

> Only var bindings can be assigned; let bindings and parameters are immutable (design §8.1).

An error from the `resolve` phase. `sakura explain V1040` prints this explanation offline.

Only `var` bindings can be assigned; `let` bindings and parameters are immutable (design §8.1).

Wrong:

```sakura
let n = 0
n = n + 1
```

Fixed:

```sakura
var n = 0
n = n + 1
```

## Wrong program [#wrong-program]

[`compiler/tests/ui/V1040_assign_immutable.sakura`](https://github.com/icantcodefyi/sakuralang/blob/master/compiler/tests/ui/V1040_assign_immutable.sakura), which the compiler's ui suite runs through `sakura check`:

```sakura
fn bump() -> Int {
  let x = 1
  x = 2
  x
}
```

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

```json
{
  "schema_version": "1.0",
  "kind": "diagnostic",
  "id": "d:0f924446648db328",
  "code": "V1040",
  "severity": "error",
  "phase": "resolve",
  "message": "cannot assign to `x`: it is bound by `let`, which is immutable",
  "snapshot": "sha256:$SNAPSHOT",
  "symbol_id": "main::V1040_assign_immutable::bump",
  "spans": [
    {
      "uri": "file://$DIR/V1040_assign_immutable.sakura",
      "content_hash": "sha256:40c9e7936c6de9f647222973e040bb9c2de3da952d0decdb81b480d2fd85436e",
      "start_byte": 93,
      "end_byte": 94,
      "role": "primary"
    },
    {
      "uri": "file://$DIR/V1040_assign_immutable.sakura",
      "content_hash": "sha256:40c9e7936c6de9f647222973e040bb9c2de3da952d0decdb81b480d2fd85436e",
      "start_byte": 85,
      "end_byte": 86,
      "role": "definition",
      "label": "bound here"
    }
  ],
  "related_ids": [],
  "expected": null,
  "actual": null,
  "obligation": null,
  "counterexample": null,
  "explanation_uri": "sakura://explain/V1040",
  "fixes": [
    {
      "id": "f:0f924446648db328.0",
      "title": "Declare it with `var`",
      "applicability": "machine_applicable",
      "impact": "local",
      "edits": [
        {
          "span": {
            "uri": "file://$DIR/V1040_assign_immutable.sakura",
            "content_hash": "sha256:40c9e7936c6de9f647222973e040bb9c2de3da952d0decdb81b480d2fd85436e",
            "start_byte": 81,
            "end_byte": 84,
            "role": "primary"
          },
          "replacement": "var"
        }
      ]
    }
  ]
}
```

## Fixed program [#fixed-program]

[`compiler/tests/ui/V1040_assign_immutable.fixed.sakura`](https://github.com/icantcodefyi/sakuralang/blob/master/compiler/tests/ui/V1040_assign_immutable.fixed.sakura), which `sakura check` accepts with no diagnostics:

```sakura
fn bump() -> Int {
  var x = 1
  x = 2
  x
}
```