# `Statifier.Document.Transition`
[🔗](https://github.com/riddler/statifier-ex/blob/v2.0.0/lib/statifier/document/transition.ex#L1)

A `<transition>` element: spec 3.5, matched against events and guarded by
a condition to move the configuration from one set of states to another.

`event` and `target` are both already-split lists (`[String.t()]`), not
the raw whitespace-separated attribute strings. `event` holds the
transition's event *descriptors* - each one still an unparsed token that
may itself use dot-separated wildcards (`"error.*"`) - and `target` holds
the tokenized target ids. The re-splitting this avoids is what
`../statifier/lib/statifier/event.ex:64-79` does on every match attempt in
v1: tokenizing once at lowering means no consumer downstream re-splits.
The *second*-level split of a descriptor into its dot-separated tokens is
not done here - that is the compiler's job, once the descriptor is being
matched against a compiled event name, not stored. An empty `event` list
means an eventless transition; an empty `target` list means a targetless
one - both spec-legal, and both distinguishable from "one descriptor" or
"one target" without a sentinel.

`cond` is raw predicator source, `String.t() | nil`, never compiled here
(`docs/datamodel.md`). Its ADR-0014 span lives at
`attribute_locations[:cond]`, the base offset ADR-0014's arithmetic adds
a within-expression span to - not a field of this struct, matching how
every other expression-bearing node stores its span.

`type` defaults to `:external`, the value lowering applies when the
attribute is absent (spec 3.5's own default). That default is applied,
not left for every consumer to re-derive, but it is not by itself proof
the author wrote `type="external"`: `attribute_locations` carries no
`:type` key when the default applied, and does when the author wrote
either value explicitly. The same rule holds for every other field with a
non-nil default in this layer.

`content` is the transition's executable content directly,
`[Document.content_node()]`, not wrapped in a `Statifier.Document.Block`:
there is no wrapping element in the source to give a block its own
location, and the transition's own `location` already is that span.

# `t`

```elixir
@type t() :: %Statifier.Document.Transition{
  attribute_locations: Statifier.Document.attribute_locations(),
  cond: String.t() | nil,
  content: [Statifier.Document.content_node()],
  event: [String.t()],
  location: Statifier.Parser.Location.t(),
  target: [String.t()],
  type: :internal | :external
}
```

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