# `Statifier.Validator.Checks.Ids`
[🔗](https://github.com/riddler/statifier-ex/blob/v2.0.0/lib/statifier/validator/checks/ids.ex#L1)

Check 1 (spec 3.14): an `ID`-typed attribute's value is unique across the
document - not only a state's `id`, but a `<data>`'s too. 3.14: "all
attributes of type "ID" MUST have unique values within an SCXML
document", and `<data>`'s `id` is type `ID` (5.3.1) - so a
`<data id="s1">` colliding with a `<state id="s1">` is exactly as much a
violation as two states sharing one id, and both join **one** uniqueness
set rather than two separate ones. `nil` ids are excluded
(`lib/statifier/document/state.ex` - a `nil` id means "the author omitted
an optional attribute", not "no id"; `<data>` has no `nil` id, since `id`
is required to build a `Data` struct at all), and document scope, not
session scope, is what this layer can see.

The **first** occurrence of a repeated id in document order is canonical
and reports nothing; every later occurrence gets its own
`{:duplicate_id, id}` error - a `<data>` colliding with an earlier
`<state>` (or vice versa) is reported at the later element's own span,
never at the canonical, earlier one.

A state or `<data>` written as `id=""` is a third case, resolved here
rather than left open: spec 3.14 types `id` as an XML Schema ID, whose
lexical space excludes the empty string, so an empty id is
`{:empty_id}` - and, like a `nil` id, it stays out of the uniqueness set
entirely. Two elements both written `id=""` are two empty ids, not a
duplicate pair; grouping them would add a third error for a collision
that is not one.

# `check`

```elixir
@spec check(
  document :: Statifier.Document.t(),
  context :: Statifier.Validator.Context.t()
) :: [
  Statifier.Validator.Error.t()
]
```

Returns an `:empty_id` error for every state or `<data>` written `id=""`
and a `:duplicate_id` error for every occurrence of a repeated non-empty
id after its first, canonical occurrence in document order. `nil` ids (an
omitted optional attribute) are excluded entirely. Returns `[]` when every
id in the document is unique or absent.

---

*Consult [api-reference.md](api-reference.md) for complete listing*