# W5005 linear_contract_on_hot_path

URL: /docs/errors/W5005
Markdown: /docs/errors/W5005.md

> A requires, ensures, where or property requires clause calls List.all, List.any, List.contains or String.contains on a parameter (or it), so every check scans the whole value (design §7.2, §17.72).

A warning from the `contract` phase. `sakura explain W5005` prints this explanation offline.

A `requires`, `ensures`, `where` or `property requires` clause calls `List.all`, `List.any`,
`List.contains` or `String.contains` on a parameter (or `it`), so every check scans the whole
value (design §7.2, §17.72). One warning per clause; std is exempt. Keep the clause when a linear
check is what you want, or move the invariant into a refined type so it is checked once, where
values are introduced.

Warning:

```sakura
fn total(xs: List<Int>) -> Int
  requires List.all(xs, fn(x) { x > 0 })
{ List.sum(xs) }
```

Fixed (the invariant is a type):

```sakura
type Positive = Int where it > 0
fn total(xs: List<Positive>) -> Int { List.fold(xs, 0, fn(acc, x) { acc + x }) }
```

## Wrong program [#wrong-program]

[`compiler/tests/ui/W5005_linear_contract_on_hot_path.sakura`](https://github.com/icantcodefyi/sakuralang/blob/master/compiler/tests/ui/W5005_linear_contract_on_hot_path.sakura), which the compiler's ui suite runs through `sakura check`:

```sakura
fn count_positive(xs: List<Int>) -> Int
  requires List.all(xs, fn(x) { x > 0 })
{ List.len(xs) }

fn many(xs: List<Int>) -> Int {
  var n = 0
  for _ in 0..1000 { n = n + count_positive(xs) }
  n
}
```

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

```json
{
  "schema_version": "1.0",
  "kind": "diagnostic",
  "id": "d:1883ce8ec9916d2c",
  "code": "W5005",
  "severity": "warning",
  "phase": "contract",
  "message": "this contract calls `List.all` on `xs`, so every check scans all of it (design §7.2)",
  "snapshot": "sha256:$SNAPSHOT",
  "symbol_id": "main::W5005_linear_contract_on_hot_path::count_positive",
  "spans": [
    {
      "uri": "file://$DIR/W5005_linear_contract_on_hot_path.sakura",
      "content_hash": "sha256:35940e368c4fd7ed2cdb57ef5c876e16d024aabc8759a11012858d797d05f8c3",
      "start_byte": 134,
      "end_byte": 163,
      "role": "primary"
    }
  ],
  "related_ids": [],
  "expected": null,
  "actual": null,
  "obligation": null,
  "counterexample": null,
  "explanation_uri": "sakura://explain/W5005",
  "fixes": []
}
```

## Fixed program [#fixed-program]

[`compiler/tests/ui/W5005_linear_contract_on_hot_path.fixed.sakura`](https://github.com/icantcodefyi/sakuralang/blob/master/compiler/tests/ui/W5005_linear_contract_on_hot_path.fixed.sakura), which `sakura check` accepts with no diagnostics:

```sakura
type Positive = Int where it > 0

fn count_positive(xs: List<Positive>) -> Int { List.len(xs) }

fn many(xs: List<Positive>) -> Int {
  var n = 0
  for _ in 0..1000 { n = n + count_positive(xs) }
  n
}
```