1 files changed, 40 insertions, 0 deletions
diff --git a/src/__OLD_parcom/optional.cr b/src/__OLD_parcom/optional.cr
new file mode 100644
index 0000000..b368d4e
--- /dev/null
+++ b/src/__OLD_parcom/optional.cr
@@ -0,0 +1,40 @@
+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
+