Implement Optional (sort of)

2 files changed, 39 insertions, 3 deletions

diff --git a/spec/parcom_spec.cr b/spec/parcom_spec.cr
index d9af3b9..2f3f6ba 100644
--- a/spec/parcom_spec.cr
+++ b/spec/parcom_spec.cr
@@ -348,7 +348,7 @@ describe Recover do
       result.value.should eq('t')
     end
 
-    it "succeeds and returns the default value without modifying the input of the wrapped parser fails" do
+    it "succeeds and returns the default value without modifying the input if the wrapped parser fails" do
       tokens = TokenStream.from_string("_____")
       result = p.parse(tokens)
 
@@ -358,7 +358,26 @@ describe Recover do
   end
 end
 
-pending Optional do
+describe Optional do
+  p = Optional.new(Token.new('t'))
+
+  describe "#parse" do
+    it "succeeds and returns the wrapped parser's value if it succeeds" do
+      tokens = TokenStream.from_string("testing")
+      result = p.parse(tokens)
+
+      result.tokens.should eq(tokens[1..])
+      result.value.should eq('t')
+    end
+
+    it "succeeds and returns a value of `nil` without modifying the input if the wrapped parser fails" do
+      tokens = TokenStream.from_string("_____")
+      result = p.parse(tokens)
+
+      result.tokens.should eq(tokens)
+      result.value.should be_nil
+    end
+  end
 end
 
 pending Tokens do
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