From 8835f1de0dbc6a059a42fad78d4acbb0984b94e2 Mon Sep 17 00:00:00 2001
From: Matthew Hall <hallmatthew314@gmail.com>
Date: Wed, 8 Mar 2023 20:03:26 +1300
Subject: Implement Plus

---
 spec/parcom_spec.cr | 50 +++++++++++++++++++++++++++++--
 src/parcom.cr       | 85 ++++++++++++++++++++++++++++++-----------------------
 2 files changed, 96 insertions(+), 39 deletions(-)

diff --git a/spec/parcom_spec.cr b/spec/parcom_spec.cr
index b2a3a80..f35465c 100644
--- a/spec/parcom_spec.cr
+++ b/spec/parcom_spec.cr
@@ -261,10 +261,56 @@ describe Map do
   end
 end
 
-pending Phrase do
+describe Plus do
+  describe "#parse" do
+    tokens = TokenStream.from_string("testing")
+    p_t = Token(Char).new('t')
+    p_e = Token(Char).new('e')
+    p_at = Token(Char).new('@')
+
+    it "fails if the first parser fails" do
+      p = p_at + p_e
+      expect_raises(ParserException) { p.parse(tokens) }
+    end
+
+    it "fails if the second parser fails" do
+      p = p_t + p_at
+      expect_raises(ParserException) { p.parse(tokens) }
+    end
+
+    it "fails if both parsers fail" do
+      p = p_at + p_at
+      expect_raises(ParserException) { p.parse(tokens) }
+    end
+
+    it "succeeds if both parsers succeed" do
+      p = p_t + p_e
+      result = p.parse(tokens)
+
+      result.tokens.should eq(tokens[2..])
+      result.value[0].should eq('t')
+      result.value[1].should eq('e')
+    end
+
+    it "evaluates parsers from left to right (left associative)" do
+      p_succeeds = p_t + p_e
+      p_fails = p_e + p_t
+
+      p_succeeds.parse(tokens) # should not raise an exception
+      expect_raises(ParserException) { p_fails.parse(tokens) }
+
+      p_s = Token(Char).new('s')
+
+      r = (p_t + p_e + p_s).parse(tokens) # should not raise an exception
+      r.value.should be_a({ {Char, Char}, Char})
+
+      r = (p_t + (p_e + p_s)).parse(tokens) # should not raise an exception
+      r.value.should be_a({Char, {Char, Char} })
+    end
+  end
 end
 
-pending Plus do
+pending Phrase do
 end
 
 pending Recover do
diff --git a/src/parcom.cr b/src/parcom.cr
index ef1aa2a..8d1d47b 100644
--- a/src/parcom.cr
+++ b/src/parcom.cr
@@ -59,15 +59,18 @@ module Parcom
       return nil
     end
 
-    def assert(f : V -> Bool)
-      Assert.new(self, f)
+    def |(other : Parser(T, V)) : Alt(T, V)
+      Alt.new(self, other)
     end
 
-    def |(other : Parser(T, V)) : Parser(T, V)
-      Alt.new(self, other)
+    def +(other : Parser(T, U)) : Plus(T, V, U) forall U
+      Plus.new(self, other)
+    end
+
+    def assert(f : V -> Bool)
+      Assert.new(self, f)
     end
 
-    # TODO: Find a way to annotate this method's type
     def map(f : V -> U) : Map(T, V, U) forall U
       Map.new(self, f)
     end
@@ -162,53 +165,61 @@ module Parcom
       Result.new(result.tokens, @f.call(result.value))
     end
   end
-end
 
-class Phrase
-end
+  class Phrase
+  end
 
-class Plus
-end
+  class Plus(T, V, U) < Parser(T, {V, U})
+    def initialize(@p1 : Parser(T, V), @p2 : Parser(T, U))
+    end
 
-class Recover
-end
+    def parse(tokens : TokenStream(T)) : Result(T, {V, U})
+      r1 = @p1.parse(tokens)
+      r2 = @p2.parse(r1.tokens)
+      Result.new(r2.tokens, {r1.value, r2.value})
+    end
+  end
 
-class Optional
-end
+  class Recover
+  end
 
-class Tokens
-end
+  class Optional
+  end
 
-class Many
-end
+  class Tokens
+  end
 
-class Some
-end
+  class Many
+  end
 
-class Exactly
-end
+  class Some
+  end
 
-class AtLeast
-end
+  class Exactly
+  end
 
-class AtMost
-end
+  class AtLeast
+  end
 
-class Between
-end
+  class AtMost
+  end
 
-class StopAt
-end
+  class Between
+  end
 
-class StopAfter
-end
+  class StopAt
+  end
 
-class StopIf
-end
+  class StopAfter
+  end
 
-class FirstOf
-end
+  class StopIf
+  end
 
-class SepBy
+  class FirstOf
+  end
+
+  class SepBy
+  end
 end
 
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