# `syntax.variable_keyword`

A variable is declared with `let`, `const` or `val`, words from other languages, where Jadpo writes `var`.

Area 3, [Logic](/docs/logic.md); not declinable; the next step is usually the agent's.

## Why Jadpo refuses it

Jadpo has one word for a variable: `var name: Type = value`, which never
changes, or `var mut` when it can. Other languages' words mean different
things from one language to the next, such as whether the value can
change, so Jadpo doesn't accept them and guess. The error names the word
to write, and the rest of the file is checked as if it were written.

## What it looks like

main.jadpo:

```jadpo
type Count = Int {
    min: 0
}

function same(count: Count) -> Count {
    let kept: Count = count
    return kept
}
```

```text
error: Jadpo writes `var`, not `let`
 --> main.jadpo:6:5
  |
6 |     let kept: Count = count
  |     ^^^ write `var`
  reason: `let` declares a variable in JavaScript, TypeScript, Swift and Rust; a Jadpo variable is `var name: Type = value`, or `var mut` when it can change.
  next: Use `var` instead of `let` (agent)
  rule: syntax.variable_keyword
  help: https://jadpo.dev/docs/errors/syntax.variable_keyword

check failed: 1 error, 1 file
```

`(agent)` at the end of the `next:` line means your coding agent can make
the fix itself.

`const` and `val` inside a body are the same error, and so is Rust's
`let mut`, whose Jadpo form is `var mut`. At the top of a file, outside any
function, `const` declares a fixed value, which Jadpo writes `constant`:

main.jadpo:

```jadpo
type Count = Int {}

const limit: Count = 3

function lim() -> Count {
    return limit
}
```

```text
error: Jadpo writes `constant`, not `const`
 --> main.jadpo:3:1
  |
3 | const limit: Count = 3
  | ^^^^^ write `constant`
  reason: `const` declares a value that never changes in JavaScript, TypeScript, Rust, Go and C#; at the top of a Jadpo file a fixed value is `constant name: Type = value`.
  next: Use `constant` instead of `const` (agent)
  rule: syntax.variable_keyword
  help: https://jadpo.dev/docs/errors/syntax.variable_keyword

check failed: 1 error, 1 file
```

## How to fix it

**If it is a variable in a body**, write `var`, or `var mut` if you assign
it again later:

```jadpo
    var kept: Count = count
```


`kept` is a variable that never changes, and the function checks.

**If it is a fixed value at the top of a file**, write `constant`:

```jadpo
constant limit: Count = 3
```


`limit` is a constant: a fixed value worked out while compiling, which
`lim` reads by name.

## Related

- [Variables](/docs/variables.md)
- [Constants](/docs/constants.md)
- [Habits from other languages](/docs/logic.md)