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require "./parser.cr"
require "./map.cr"
module Parcom
# `Left` is a `Parser` that tries to parse with two different
# parsers in succession and fails if either of the two parsers fails.
# This parser behaves the same as `Plus`, but only returns the result
# of the first parser.
class Left(T, V, U) < Parser(T, V)
@p : Map(T, {V, U}, V)
# Accepts the two parsers to use, in order.
def initialize(p1 : Parser(T, V), p2 : Parser(T, U))
@p = (p1 + p2).map(&.first)
end
# Tries to parse with the two given parsers, and returns the
# result of the first parser if they both succeed.
def parse(tokens : Tokens(T)) : Result(T, V)
@p.parse(tokens)
rescue ex : ParserFail
raise ParserFail.new("Left: #{ex.message}")
end
end
end
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