From 658876fa4db1bb8363a03135fd22ad61b25050b7 Mon Sep 17 00:00:00 2001
From: Matthew Hall <hallmatthew314@gmail.com>
Date: Fri, 17 Mar 2023 23:54:46 +1300
Subject: recover and optional

---
 spec/parcom_spec.cr  | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 src/parcom/parser.cr | 21 ++++++++++++++++++---
 2 files changed, 65 insertions(+), 3 deletions(-)

diff --git a/spec/parcom_spec.cr b/spec/parcom_spec.cr
index 14e232e..6acd1b2 100644
--- a/spec/parcom_spec.cr
+++ b/spec/parcom_spec.cr
@@ -239,5 +239,52 @@ describe Parser do
       expect_raises(ParserFail) { p.parse(tokens) }
     end
   end
+
+  describe "#recover" do
+    p = Parser(Char, Char).token('a').recover('z')
+
+    it "behaves as normal on success" do
+      tokens = Tokens.from_string("abcd")
+      result = p.parse(tokens)
+
+      result.value.should eq('a')
+      result.tokens.should eq(tokens[1..])
+    end
+
+    tokens2 = Tokens.from_string("bbcd")
+    result2 = p.parse(tokens2)
+
+    it "returns the default value instead of failing" do
+      result2.value.should eq('z')
+    end
+
+    it "does not modify the input when recovering" do
+      result2.tokens.should eq(tokens2)
+    end
+  end
+
+  describe "#optional" do
+    p = Parser(Char, Char).token('a').optional
+
+    it "behaves as normal on success" do
+      tokens = Tokens.from_string("abcd")
+      result = p.parse(tokens)
+
+      result.value.should eq('a')
+      result.tokens.should eq(tokens[1..])
+    end
+
+    tokens2 = Tokens.from_string("bbcd")
+    result2 = p.parse(tokens2)
+
+    it "returns nil instead of failing" do
+      result2.value.should be_nil
+    end
+
+    it "does not modify the input when recovering" do
+      result2.tokens.should eq(tokens2)
+    end
+
+  end
 end
 
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
 
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