# `Grammar.IR.Custom`
[🔗](https://github.com/joetjen/ichor/blob/v0.3.0/lib/grammar/ir/custom.ex#L1)

`@native("Module", "function", dep1, dep2, ...)`: a rule body that
delegates to hand-written Elixir code (an `Ichor.CustomRule`
implementation) instead of ordinary `Grammar.IR` combinators -- the
escape hatch for constructs no static PEG grammar can express (a
Prolog-style mutable operator-precedence table, C's typedef-vs-
expression ambiguity, and similar "what matches here depends on
something read earlier" problems).

`deps` names the only other rules `module`/`function` is allowed to
call back into (via the `rule_matchers` map `c:Ichor.CustomRule.match/4`
receives) -- keeping this node's dependencies visible to
`Grammar.Analysis`'s reference check even though its own body is
opaque. `nullable`/`leading` are author-supplied facts standing in for
what `Grammar.Analysis` would otherwise compute structurally (it can't,
since the body is arbitrary code): `nullable` defaults to `false`
(matches the common case -- most custom recognizers consume
something), `leading` defaults to `deps` (conservative -- assumes it
might call any declared dependency as its very first, unconsumed
action, for left-recursion-cycle detection).

# `t`

```elixir
@type t() :: %Grammar.IR.Custom{
  deps: [atom()],
  function: atom(),
  leading: [atom()],
  meta: Grammar.IR.Meta.t(),
  module: module(),
  nullable: boolean()
}
```

---

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