# V3011 handler_clause_mismatch

URL: /docs/errors/V3011
Markdown: /docs/errors/V3011.md

> A handler must provide exactly one clause per operation of its effect, with matching parameter counts (design §5.5).

An error from the `effect` phase. `sakura explain V3011` prints this explanation offline.

A `handler` must provide exactly one clause per operation of its effect, with matching
parameter counts (design §5.5). Add the missing clause or remove the extra one.

Wrong:

```sakura
effect Pair { fn left() -> Int; fn right() -> Int }
fn only_left() -> Handler<Pair> { handler Pair { fn left() { 1 } } }
```

Fixed:

```sakura
fn both() -> Handler<Pair> { handler Pair { fn left() { 1 }; fn right() { 2 } } }
```

## Wrong program [#wrong-program]

[`compiler/tests/ui/V3011_handler_clause_mismatch.sakura`](https://github.com/icantcodefyi/sakuralang/blob/master/compiler/tests/ui/V3011_handler_clause_mismatch.sakura), which the compiler's ui suite runs through `sakura check`:

```sakura
effect Pair {
  fn left() -> Int
  fn right() -> Int
}

fn only_left() -> Handler<Pair> {
  handler Pair {
    fn left() { 1 }
  }
}
```

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

```json
{
  "schema_version": "1.0",
  "kind": "diagnostic",
  "id": "d:1be4850aef641f24",
  "code": "V3011",
  "severity": "error",
  "phase": "effect",
  "message": "handler for `Pair` is missing a clause for `right`",
  "snapshot": "sha256:$SNAPSHOT",
  "symbol_id": "main::V3011_handler_clause_mismatch::only_left",
  "spans": [
    {
      "uri": "file://$DIR/V3011_handler_clause_mismatch.sakura",
      "content_hash": "sha256:97dd07d17fafd8305acbdc5c3f9d2e754f8f7d4664743a1acc04cafc1e52e2bd",
      "start_byte": 126,
      "end_byte": 164,
      "role": "primary"
    }
  ],
  "related_ids": [],
  "expected": null,
  "actual": null,
  "obligation": null,
  "counterexample": null,
  "explanation_uri": "sakura://explain/V3011",
  "fixes": []
}
```

## Fixed program [#fixed-program]

[`compiler/tests/ui/V3011_handler_clause_mismatch.fixed.sakura`](https://github.com/icantcodefyi/sakuralang/blob/master/compiler/tests/ui/V3011_handler_clause_mismatch.fixed.sakura), which `sakura check` accepts with no diagnostics:

```sakura
effect Pair {
  fn left() -> Int
  fn right() -> Int
}

fn both() -> Handler<Pair> {
  handler Pair {
    fn left() { 1 }
    fn right() { 2 }
  }
}
```