# `Statifier.Effect.Invoke`
[🔗](https://github.com/riddler/statifier-ex/blob/v2.0.0/lib/statifier/effect/invoke.ex#L1)

Payload for `{:invoke, %__MODULE__{}}` - spec 6.4's `<invoke>`. Fields are
the element's own attribute names: `invoke_id` (`id`/`idlocation`,
generated when the element has none), `type`, `src`, `params` (the
resolved `<param>`/namelist payload), `content` (the resolved `<content>`
payload), `autoforward` (the `autoforward` attribute).

`<invoke>` is attached to a state, not to a block of executable content,
so this payload carries `state_index` - the invoking state's index
(constraint 3) - rather than a `c_index`; there is no content node to
identify. `state_index` is named apart from `src` (spec 6.4's URI
attribute) on purpose - the two fields are two letters apart with
unrelated meanings, and the original `source` name invited confusing them
(post-review correction).
`macrostep`/`microstep`/`round` are the counters as they stand when the
invoke is produced.

`invoke_index` is the invoking state's `invoke` list position (document
order) - the second half of `Statifier.Event.Cause.origin()`'s
`{:invoke, state_index, invoke_index}` identity, which the session needs
to name the failing element in an `error.communication` raised when it
cannot start the child (spec 3.12.2). `invoke_one/6` already has the value
in scope when it builds this struct; it was dropped rather than carried
until this payload had a caller for it, the same "no dead field"
discipline `origin`/`sendid` on `Statifier.Event` followed.

## `src` and `content` are two fields, not one

Spec 6.4 collapses the data channels an invoked service can receive to two
pairs treated identically: "these services MUST treat values specified by
`<param>` and namelist identically" and "MUST also treat values specified
by 'src' and `<content>` identically." The first pair shares one field
(`params`) because both lower to the same `Statifier.Machine.Param.t()`
shape and coerce through the same `Statifier.EventData.coerce({:params,
_})` ladder. The second pair cannot: `src` is a URI string the core never
dereferences (ADR-0031 - whether anything downstream fetches it is a later
concern), while `content` is markup or a value already resolved in hand,
coerced through `Statifier.EventData.coerce({:value, _})` or `{:text, _}`.
One field cannot hold both a reference and a resolved value without a
sentinel that collapses "no content, use src" with "content resolved to
nil" - so 6.4's "treat identically" becomes two fields the reader of the
effect payload treats identically, not one field that already has.

# `t`

```elixir
@type t() :: %Statifier.Effect.Invoke{
  autoforward: boolean() | nil,
  content: term(),
  invoke_id: String.t(),
  invoke_index: non_neg_integer(),
  macrostep: non_neg_integer(),
  microstep: non_neg_integer(),
  params: term(),
  round: non_neg_integer(),
  src: String.t() | nil,
  state_index: non_neg_integer(),
  type: String.t() | nil
}
```

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