# V1020 recursive_lambda

URL: /docs/errors/V1020
Markdown: /docs/errors/V1020.md

> Local lambdas cannot refer to themselves.

An error from the `resolve` phase. `sakura explain V1020` prints this explanation offline.

Local lambdas cannot refer to themselves. Lift the function to the top level, where functions
may be mutually recursive.

Wrong:

```sakura
let fact = fn(n) { if n == 0 { 1 } else { n * fact(n - 1) } }
```

Fixed:

```sakura
fn fact(n: Int) -> Int { if n == 0 { 1 } else { n * fact(n - 1) } }
```

## Wrong program [#wrong-program]

[`compiler/tests/ui/V1020_recursive_lambda.sakura`](https://github.com/icantcodefyi/sakuralang/blob/master/compiler/tests/ui/V1020_recursive_lambda.sakura), which the compiler's ui suite runs through `sakura check`:

```sakura
fn fact_of_five() -> Int {
  let fact = fn(n) { if n == 0 { 1 } else { n * fact(n - 1) } }
  fact(5)
}
```

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

```json
{
  "schema_version": "1.0",
  "kind": "diagnostic",
  "id": "d:296aa8452551a950",
  "code": "V1020",
  "severity": "error",
  "phase": "resolve",
  "message": "local lambda `fact` cannot call itself; lift it to a top-level `fn` (design §4.5)",
  "snapshot": "sha256:$SNAPSHOT",
  "symbol_id": "main::V1020_recursive_lambda::fact_of_five",
  "spans": [
    {
      "uri": "file://$DIR/V1020_recursive_lambda.sakura",
      "content_hash": "sha256:b3195773f224d3690fe85396ada7c0c4b7be8a53a36bdc3cd1e635e61d74760b",
      "start_byte": 109,
      "end_byte": 113,
      "role": "primary"
    }
  ],
  "related_ids": [],
  "expected": null,
  "actual": null,
  "obligation": null,
  "counterexample": null,
  "explanation_uri": "sakura://explain/V1020",
  "fixes": []
}
```

## Fixed program [#fixed-program]

[`compiler/tests/ui/V1020_recursive_lambda.fixed.sakura`](https://github.com/icantcodefyi/sakuralang/blob/master/compiler/tests/ui/V1020_recursive_lambda.fixed.sakura), which `sakura check` accepts with no diagnostics:

```sakura
fn fact(n: Int) -> Int { if n == 0 { 1 } else { n * fact(n - 1) } }

fn fact_of_five() -> Int { fact(5) }
```