From 27fb4fa0babc2c3db41c93865a35430515ff3630 Mon Sep 17 00:00:00 2001
From: Matthew Hall <hallmatthew314@gmail.com>
Date: Sat, 11 Mar 2023 00:35:43 +1300
Subject: Implement Left and Right parsers

---
 spec/parcom_spec.cr | 32 ++++++++++++++++++++++++++++++++
 src/parcom.cr       | 36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+)

diff --git a/spec/parcom_spec.cr b/spec/parcom_spec.cr
index 77a14dd..d67f19a 100644
--- a/spec/parcom_spec.cr
+++ b/spec/parcom_spec.cr
@@ -308,6 +308,38 @@ describe Plus do
   end
 end
 
+# most behavior shouldn't need to be tested
+# since it is based on tested bbehavior from
+# Plus and Map
+describe Left do
+  describe "#parse" do
+    it "returns the value of the first parser if both succeed" do
+      tokens = TokenStream.from_string("testing")
+      letter_t = Token.new('t')
+      letter_e = Token.new('e')
+      result = (letter_t << letter_e).parse(tokens)
+
+      result.value.should eq('t')
+      result.tokens.should eq(tokens[2..])
+    end
+  end
+end
+
+# same deal as Left
+describe Right do
+  describe "#parse" do
+    it "returns the value of the second parser if both succeed" do
+      tokens = TokenStream.from_string("testing")
+      letter_t = Token.new('t')
+      letter_e = Token.new('e')
+      result = (letter_t >> letter_e).parse(tokens)
+
+      result.value.should eq('e')
+      result.tokens.should eq(tokens[2..])
+    end
+  end
+end
+
 describe Phrase do
   p = Phrase.new(Token.new('t'))
 
diff --git a/src/parcom.cr b/src/parcom.cr
index 2a61c2d..f0295a3 100644
--- a/src/parcom.cr
+++ b/src/parcom.cr
@@ -67,6 +67,14 @@ module Parcom
       Plus.new(self, other)
     end
 
+    def <<(other : Parser(T, U)) : Left(T, V, U) forall U
+      Left.new(self, other)
+    end
+
+    def >>(other : Parser(T, U)) : Right(T, V, U) forall U
+      Right.new(self, other)
+    end
+
     def assert(f : V -> Bool)
       Assert.new(self, &f)
     end
@@ -233,6 +241,34 @@ module Parcom
     end
   end
 
+  class Left(T, V, U) < Parser(T, V)
+    @p : Map(T, {V, U}, V)
+
+    def initialize(p1 : Parser(T, V), p2 : Parser(T, U))
+      @p = (p1 + p2).map(&.first)
+    end
+
+    def parse(tokens : TokenStream(T)) : Result(T, V)
+      @p.parse(tokens)
+    rescue ex : ParserException
+      raise ParserException.new("Left: #{ex.message}")
+    end
+  end
+
+  class Right(T, V, U) < Parser(T, U)
+    @p : Map(T, {V, U}, U)
+
+    def initialize(p1 : Parser(T, V), p2 : Parser(T, U))
+      @p = (p1 + p2).map(&.last)
+    end
+
+    def parse(tokens : TokenStream(T)) : Result(T, U)
+      @p.parse(tokens)
+    rescue ex : ParserException
+      raise ParserException.new("Right: #{ex.message}")
+    end
+  end
+
   class Recover(T, V) < Parser(T, V)
     @p : Map(T, V?, V)
 
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