You're seeing just the function satisfy, go back to Ergo.Combinators module for more information.
Link to this function

satisfy(parser, pred_fn, opts \\ [])

The satisfy/3 parser takes a parser and a predicate function. If the parser is successful the AST is passed to the predicate function. If the predicate function returns true the parser returns the successful context, otherwise an error context is returned.


iex> alias Ergo.Context
iex> import Ergo.{Terminals, Combinators, Numeric}
iex> parser = satisfy(any(), fn char -> char in (?0..?9) end, label: "digit char")
iex> assert %Context{status: :ok, ast: ?4} = Ergo.parse(parser, "4")
iex> assert %Context{status: {:error, [{:unsatisfied, "Failed to satisfy: digit char"}]}} = Ergo.parse(parser, "!")
iex> parser = satisfy(number(), fn n -> Integer.mod(n, 2) == 0 end, label: "even number")
iex> assert %Context{status: :ok, ast: 42} = Ergo.diagnose(parser, "42")
iex> assert %Context{status: {:error, [{:unsatisfied, "Failed to satisfy: even number"}]}} = Ergo.parse(parser, "27")