The hoist/1 parser takes a parser expected to return an AST which is a 1-item list. The returned parser extracts the item from the list and returns an AST of just that item.

This often comes up with the sequence/2 parser and ignore, where all but one item in a sequence are ignored. Using hoist pulls that item up so that subsequent parsers don't need to deal with the list.

Examples

iex> alias Ergo
iex> alias Ergo.Context
iex> import Ergo.{Terminals, Combinators}
iex> parser = sequence([ignore(many(char(?a))), char(?b)]) |> hoist()
iex> assert %Context{status: :ok, ast: ?b} = Ergo.parse(parser, "aaaaaaaab")