recover and optional

1 files changed, 18 insertions, 3 deletions

diff --git a/src/parcom/parser.cr b/src/parcom/parser.cr
index 3098080..b78ab77 100644
--- a/src/parcom/parser.cr
+++ b/src/parcom/parser.cr
@@ -169,9 +169,24 @@ module Parcom
     # Creates a new parser that is the same as the parser object it is
     # called from, but it will return a default value without consuming
     # any input instead of failing.
-    #def recover(default : U) : Parser(T, U)
-    #  nil
-    #end
+    def recover(default : U) : Parser(T, U)
+      r = Parser(T, U).pure(default)
+      (self | r).named("#{@name} (recover <#{default}>)")
+    end
+
+    # Creates a new parser that is the same  as the parser object it is
+    # called from, but it will return `nil` without consuming any input
+    # instead of returning.
+    def optional : Parser(T, U?)
+      p = self
+      Parser(T, U?).new("#{p.name} (optional)") do |tokens|
+        # We have to do it this way due to how type unions work.
+        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
 end