require "./parser.cr"
require "./any_token.cr"
require "./assert.cr"

module Parcom
  # `Satisfy` is a `Parser` that parses a single token
  # if that token passes a predefined test, similar
  # to `Assert`. This class is effectively a shorthand
  # for the following:
  # ```
  # # These parsers are equivalent.
  # letter_assert = Assert.new(AnyToken(Char).new) { |x| x.letter? }
  # letter_satisfy = Satisfy(Char).new { |x| x.letter? }
  # ```
  class Satisfy(T) < Parser(T, T)
    @p : Assert(T, T)

    # Accepts the `Bool`-returning block containing the test
    # to run on the parsed token.
    def initialize(&block : T -> Bool)
      @p = AnyToken(T).new.assert(&block)
    end

    # Returns the first token of the input if that
    # token passes the test.
    def parse(tokens : Tokens(T)) : Result(T, T)
      @p.parse(tokens)
    rescue ex : ParserFail
      raise ParserFail.new("Satisfy: #{ex.message}")
    end
  end
end