---
title: "Name a reusable constraint"
description: "Build a `uint8`/`port` vocabulary as a `type()`-marked block of ordinary fields."
source: "https://aontu.dev/how-to/name-a-reusable-constraint/"
---

# Name a reusable constraint

Build a \`uint8\`/\`port\` vocabulary as a \`type()\`-marked block of ordinary fields.

Rendered from [`docs/how-to/name-a-reusable-constraint.md`](https://github.com/aontu-lang/aontu/blob/main/docs/how-to/name-a-reusable-constraint.md) in the engine repository, where a correction belongs, and where the test suite executes every example on this page.

aontu has no `uint8`, `int8` or `port` keyword, and does not need one: a constraint is a value, so a name for one is a field. A `type()`\-marked block gives you a library of them that unifies like everything else and emits nothing:

```aontu
type: type({})

type: {
  uint8: integer & min(0) & max(255)
  int8: integer & min(-128) & max(127)
  port: integer & min(1) & max(65535)
}

listen: $.type.port
listen: 8080
```

```json
{ "listen": 8080 }
```

The block is absent from the output and present for unification, and the key name is not reserved: `type` here is a field that happens to be `type()`\-marked, so `defs` or `schema` reads the same.

An out-of-range value is refused at the field that holds it. Write the same document with `listen: 70000` as `types.aon`:

```aontu
type: type({})

type: {
  uint8: integer & min(0) & max(255)
  int8: integer & min(-128) & max(127)
  port: integer & min(1) & max(65535)
}

listen: $.type.port
listen: 70000
```

```sh
$ aontu types.aon
[aontu/constraint]: Cannot unify values at path $.listen
...
 Cannot unify value: 70000 with value: integer&min(1)&max(65535)
...
$ echo $?
1
```

The refusal states the normalised residual the value had to satisfy, not the alias’s name: the constraint travelled, the label did not.

Lead with the kind. `min(0) & max(255)` alone bounds a _number_, so `1.5` satisfies it:

```aontu
loose: type({})
loose: byteish: min(0) & max(255)
a: $.loose.byteish
a: 1.5
```

```json
{ "a": 1.5 }
```

A sized integer is `integer & min(0) & max(255)`, which is why every alias above starts with `integer`.

Aliases are ordinary values, so they compose: one can be defined in terms of another, and a use site can narrow one further (`$.type.uint8 & max(15)`). For a file-local name that skips the path entirely, `%port = integer & min(1) & max(65535)` declares an [alias](https://aontu.dev/docs/reference-language#aliases-) used as bare `%port`.

The idiom is specified in [Named constraint aliases](https://aontu.dev/docs/reference-language#named-constraint-aliases); the `type()` mark’s mechanics are the subject of [keep schema out of the output](https://aontu.dev/how-to/keep-schema-out-of-output).