# V2005 infinite_type

URL: /docs/errors/V2005
Markdown: /docs/errors/V2005.md

> Inference would need a type that contains itself (the occurs check failed).

An error from the `type` phase. `sakura explain V2005` prints this explanation offline.

Inference would need a type that contains itself (the occurs check failed). Usually a function
was applied to itself or a value was put inside its own list; add an annotation to find where.

Wrong:

```sakura
let _ = fn(x) { x(x) }
```

Fixed:

```sakura
let _ = fn(x: Int) { x }
```

## Wrong program [#wrong-program]

[`compiler/tests/ui/V2005_infinite_type.sakura`](https://github.com/icantcodefyi/sakuralang/blob/master/compiler/tests/ui/V2005_infinite_type.sakura), which the compiler's ui suite runs through `sakura check`:

```sakura
fn omega() -> Unit { let _ = fn(x) { x(x) } }
```

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

```json
{
  "schema_version": "1.0",
  "kind": "diagnostic",
  "id": "d:4d6248d8fec80f0c",
  "code": "V2005",
  "severity": "error",
  "phase": "type",
  "message": "this expression would need an infinite type: `?A` = `fn(?A) -> ?B ! ?e1`",
  "snapshot": "sha256:$SNAPSHOT",
  "symbol_id": "main::V2005_infinite_type::omega",
  "spans": [
    {
      "uri": "file://$DIR/V2005_infinite_type.sakura",
      "content_hash": "sha256:27f65a7229749e5131e2a3337167b8619997bac0ab6f4964d97034c905723ed7",
      "start_byte": 71,
      "end_byte": 75,
      "role": "primary",
      "label": "this has type `fn(?A) -> ?B ! ?e1`"
    }
  ],
  "related_ids": [],
  "expected": {
    "render": "?A"
  },
  "actual": {
    "render": "fn(?A) -> ?B ! ?e1"
  },
  "obligation": null,
  "counterexample": null,
  "explanation_uri": "sakura://explain/V2005",
  "fixes": []
}
```

## Fixed program [#fixed-program]

[`compiler/tests/ui/V2005_infinite_type.fixed.sakura`](https://github.com/icantcodefyi/sakuralang/blob/master/compiler/tests/ui/V2005_infinite_type.fixed.sakura), which `sakura check` accepts with no diagnostics:

```sakura
fn omega() -> Unit { let _ = fn(x: Int) { x } }
```