From 265b25d5f4cd44d0912f860a66e32186bf163594 Mon Sep 17 00:00:00 2001
From: Torsten Ruger <torsten@villataika.fi>
Date: Fri, 14 Apr 2017 21:01:50 +0300
Subject: [PATCH] introduce return_sequence instruction to mom

---
 lib/mom/instruction.rb                  |  1 +
 lib/mom/return_sequence.rb              | 24 ++++++++++++++++++++++++
 lib/vool/statements/return_statement.rb |  6 +++---
 test/vool/to_mom/test_return.rb         |  8 +++++++-
 4 files changed, 35 insertions(+), 4 deletions(-)
 create mode 100644 lib/mom/return_sequence.rb

diff --git a/lib/mom/instruction.rb b/lib/mom/instruction.rb
index 5782d144..0c3609cf 100644
--- a/lib/mom/instruction.rb
+++ b/lib/mom/instruction.rb
@@ -7,3 +7,4 @@ module Mom
 end
 
 require_relative "slot_load"
+require_relative "return_sequence"
diff --git a/lib/mom/return_sequence.rb b/lib/mom/return_sequence.rb
new file mode 100644
index 00000000..7850a7cd
--- /dev/null
+++ b/lib/mom/return_sequence.rb
@@ -0,0 +1,24 @@
+module Mom
+
+  # The ReturnSequence models the return from a method.
+  #
+  # This involves the jump to the return address stored in the message, and
+  # the reinstantiation of the previous message.
+  #
+  # The machine (mom) only ever "knows" one message, the current message.
+  # Messages are a double linked list, calling involves going forward,
+  # returning means going back.
+  #
+  # The return value of the current message is transferred into the return value of the
+  # callers return value during the swap of messages, and just before the jump.
+  #
+  # The callers perspective of a call is the magical apperance of a return_value
+  # in it's message at the instruction after the call.
+  #
+  # The instruction is not parameterized as it translates to a constant
+  # set of lower level instructions.
+  #
+  class ReturnSequence < Instruction
+  end
+
+end
diff --git a/lib/vool/statements/return_statement.rb b/lib/vool/statements/return_statement.rb
index 5f33895e..076691a7 100644
--- a/lib/vool/statements/return_statement.rb
+++ b/lib/vool/statements/return_statement.rb
@@ -13,10 +13,10 @@ module Vool
 
     # To return form a method in mom instructions we need to do three things:
     # - store the given return value, this is a SlotMove / SlotConstant
-    # - restore the previous message
-    # - jump to the return address
+    # - activate return sequence (reinstantiate old message and jump to return address)
     def to_mom( method )
-      [Mom::SlotConstant.new([:message , :return_value] , @return_value)]
+      [Mom::SlotConstant.new([:message , :return_value] , @return_value) ,
+      Mom::ReturnSequence.new]
     end
 
   end
diff --git a/test/vool/to_mom/test_return.rb b/test/vool/to_mom/test_return.rb
index 27b07756..43a08259 100644
--- a/test/vool/to_mom/test_return.rb
+++ b/test/vool/to_mom/test_return.rb
@@ -15,10 +15,16 @@ module Vool
     def test_slot_is_set
       assert @stats.first.left
     end
+    def test_two_instructions_are_returned
+      assert_equal 2 ,  @stats.length
+    end
+    def test_second_is_return
+      assert_equal Mom::ReturnSequence,  @stats.last.class
+    end
     def test_slot_starts_at_message
       assert_equal :message , @stats.first.left[0]
     end
-    def test_slot_gets_self
+    def test_slot_gets_return
       assert_equal :return_value , @stats.first.left[1]
     end
     def test_slot_assigns_something