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require "./parser.cr"
module Parcom
# `FirstOf` is a `Parser` that accepts multiple parsers, and tries to parse
# with all of them, in order. As soon as one of the parsers succeeds,
# that parser's result is returned. If none of the parsers are successful,
# the parsing fails.
class FirstOf(T, V) < Parser(T, V)
@p : Parser(T, V)
# Accepts the parsers to use. Raises an `ArgumentError` if
# no parsers are provided.
def initialize(ps : Iterable(Parser(T, V)))
ps_iter = ps.each
p = ps_iter.next
if p.is_a?(Iterator::Stop)
raise ArgumentError.new("FirstOf requires atleast one parser, got none")
end
# Combine all the parsers into one by wrapping them with `Alt`.
@p = p
loop do
p = ps_iter.next
break if p.is_a?(Iterator::Stop)
@p = @p | p
end
end
# Tries to parse with each of the given parsers. Either returns the first
# successful result or fails if no parsers succeed.
def parse(tokens : Tokens(T)) : Result(T, V)
@p.parse(tokens)
rescue ex : ParserFail
raise ParserFail.new("FirstOf: #{ex.message}")
end
end
end
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