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require "./parser.cr"
module Parcom
# `Optional` is a `Parser` that tries to parse with another parser,
# but does not fail the parser chain if the wrapped parser fails.
# If the wrapped parser fails, an `Optional` will not modify the input
# stream, and return `nil` instead.
#
# Example:
# ```
# digit = Satisfy(Char).new { |c| c.digit? }
# digits = Many.new(digit)
# plus_sign = Optional.new(Token.new('+'))
# positive_int = plus_sign >> digits
#
# input1 = Tokens.from_string("+12345")
# input2 = Tokens.from_string("12345")
#
# # Both of these has the same result
# positive_int.parse(input1)
# positive_int.parse(input2)
# ```
class Optional(T, V) < Parser(T, V?)
# Accepts the parser to use.
def initialize(@p : Parser(T, V))
end
# Tries to parse with the given parser, succeeds with `nil`
# instead of failing. This parser does not consume input if
# the wrapped parser fails.
def parse(tokens : Tokens(T)) : Result(T, V?)
r = @p.parse?(tokens)
new_tokens = r.nil? ? tokens : r.tokens
new_value = r.nil? ? nil : r.value
Result.new(new_tokens, new_value)
end
end
end
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