Implement Optional (sort of)

1 files changed, 18 insertions, 1 deletions

diff --git a/src/parcom.cr b/src/parcom.cr
index f102728..87c73c5 100644
--- a/src/parcom.cr
+++ b/src/parcom.cr
@@ -201,7 +201,24 @@ module Parcom
     end
   end
 
-  class Optional
+  # TODO: find a way to make this use Recover on the back end
+  class Optional(T, V) < Parser(T, V?)
+    def initialize(@p : Parser(T, V))
+    end
+
+    def parse(tokens : TokenStream(T)) : Result(T, V?)
+      r = @p.parse?(tokens)
+
+      if r.nil?
+        Result.new(tokens, nil)
+      else
+        Result.new(r.tokens, r.value || nil)
+      end
+
+      new_tokens = r.nil? ? tokens : r.tokens
+      new_value = r.nil? ? nil : r.value
+      Result.new(new_tokens, new_value)
+    end
   end
 
   class Tokens