# V3001 effect_not_allowed

URL: /docs/errors/V3001
Markdown: /docs/errors/V3001.md

> The body performs an effect that the function's signature does not allow.

An error from the `effect` phase. `sakura explain V3001` prints this explanation offline.

The body performs an effect that the function's signature does not allow. A signature's effect
row is an upper bound (design §5.7); omitting `!` means `! {}`.

The `cause` span points at the call that introduced the effect. Two repairs:
- add the effect to the signature (`interface_change`: every caller must allow it too), or
- handle it locally with `with handler { … }` (`local`).

Wrong:

```sakura
fn greet(name: String) -> Unit { Console.print("hello, {name}") }
```

Fixed:

```sakura
fn greet(name: String) -> Unit ! {Console} { Console.print("hello, {name}") }
```

## Wrong program [#wrong-program]

[`compiler/tests/ui/V3001_greet_without_console.sakura`](https://github.com/icantcodefyi/sakuralang/blob/master/compiler/tests/ui/V3001_greet_without_console.sakura), which the compiler's ui suite runs through `sakura check`:

```sakura
fn greet(name: String) -> Unit { Console.print("hello, {name}") }

pub fn main() -> Unit ! {Console} { greet("Sakura Lang") }
```

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

```json
{
  "schema_version": "1.0",
  "kind": "diagnostic",
  "id": "d:2d3bc08f407f66bb",
  "code": "V3001",
  "severity": "error",
  "phase": "effect",
  "message": "function `greet` performs `Console`, which its signature does not allow (allowed: `{}`)",
  "snapshot": "sha256:$SNAPSHOT",
  "symbol_id": "main::V3001_greet_without_console::greet",
  "spans": [
    {
      "uri": "file://$DIR/V3001_greet_without_console.sakura",
      "content_hash": "sha256:167ca72fd1e1d0fc9cd0d2f54661186c5db27ec82383560357de1a360f630b68",
      "start_byte": 70,
      "end_byte": 75,
      "role": "primary"
    },
    {
      "uri": "file://$DIR/V3001_greet_without_console.sakura",
      "content_hash": "sha256:167ca72fd1e1d0fc9cd0d2f54661186c5db27ec82383560357de1a360f630b68",
      "start_byte": 100,
      "end_byte": 130,
      "role": "cause",
      "label": "`Console.print` performs `Console` here"
    }
  ],
  "related_ids": [],
  "expected": {
    "render": "{}",
    "row": {
      "labels": [],
      "tail": null
    }
  },
  "actual": {
    "render": "{Console}",
    "row": {
      "labels": [
        {
          "effect": "std::Console",
          "args": []
        }
      ],
      "tail": null
    }
  },
  "obligation": null,
  "counterexample": null,
  "explanation_uri": "sakura://explain/V3001",
  "fixes": [
    {
      "id": "f:2d3bc08f407f66bb.0",
      "title": "Add `Console` to the signature",
      "applicability": "suggested",
      "impact": "interface_change",
      "edits": [
        {
          "span": {
            "uri": "file://$DIR/V3001_greet_without_console.sakura",
            "content_hash": "sha256:167ca72fd1e1d0fc9cd0d2f54661186c5db27ec82383560357de1a360f630b68",
            "start_byte": 97,
            "end_byte": 97,
            "role": "primary"
          },
          "replacement": " ! {Console}"
        }
      ]
    },
    {
      "id": "f:2d3bc08f407f66bb.1",
      "title": "Handle the effect locally with `with`",
      "applicability": "incomplete",
      "impact": "local",
      "edits": [
        {
          "span": {
            "uri": "file://$DIR/V3001_greet_without_console.sakura",
            "content_hash": "sha256:167ca72fd1e1d0fc9cd0d2f54661186c5db27ec82383560357de1a360f630b68",
            "start_byte": 98,
            "end_byte": 98,
            "role": "primary"
          },
          "replacement": "{ with ?handler "
        },
        {
          "span": {
            "uri": "file://$DIR/V3001_greet_without_console.sakura",
            "content_hash": "sha256:167ca72fd1e1d0fc9cd0d2f54661186c5db27ec82383560357de1a360f630b68",
            "start_byte": 132,
            "end_byte": 132,
            "role": "primary"
          },
          "replacement": " }"
        }
      ]
    }
  ]
}
```

## Fixed program [#fixed-program]

[`compiler/tests/ui/V3001_greet_without_console.fixed.sakura`](https://github.com/icantcodefyi/sakuralang/blob/master/compiler/tests/ui/V3001_greet_without_console.fixed.sakura), which `sakura check` accepts with no diagnostics:

```sakura
fn greet(name: String) -> Unit ! {Console} { Console.print("hello, {name}") }

pub fn main() -> Unit ! {Console} { greet("Sakura Lang") }
```