Make behaviour calls return None (RFC 28) (#1460)

Closes #1455.

examples/yield/main.pony

@@ -220,20 +220,20 @@ actor Main
       match punk
       | true =>
         PunkDemo(env)
-          .loop()
-          .inc().inc().inc()
-          .dec().dec().dec()
-          .inc().dec()
-          .kill()
+          .>loop()
+          .>inc().>inc().>inc()
+          .>dec().>dec().>dec()
+          .>inc().>dec()
+          .>kill()
       else
         match perf > 0
         | true =>
           InterruptiblePony(env,debug,perf).perf()
           LonelyPony(env,debug,perf).perf()
         else
           match lonely
-          | false => InterruptiblePony(env,debug).forever().kill()
-          | true => LonelyPony(env,debug).forever().kill()
+          | false => InterruptiblePony(env,debug).>forever().>kill()
+          | true => LonelyPony(env,debug).>forever().>kill()
           end
         end
       end

src/libponyc/codegen/codegen.c

@@ -762,6 +762,9 @@ static bool init_module(compile_t* c, ast_t* program, pass_opt_t* opt)
   c->reach = reach_new();
   c->tbaa_mds = tbaa_metadatas_new();
 
+  // This gets a real value once the instance of None has been generated.
+  c->none_instance = NULL;
+
   return true;
 }
 

src/libponyc/codegen/codegen.h

@@ -154,6 +154,7 @@ typedef struct compile_t
   LLVMValueRef tbaa_root;
   LLVMValueRef tbaa_descriptor;
   LLVMValueRef tbaa_descptr;
+  LLVMValueRef none_instance;
 
   LLVMTypeRef void_type;
   LLVMTypeRef ibool;

src/libponyc/codegen/gencall.c

@@ -670,7 +670,17 @@ LLVMValueRef gen_call(compile_t* c, ast_t* ast)
     codegen_debugloc(c, ast);
     gen_send_message(c, m, args, positional);
     codegen_debugloc(c, NULL);
-    r = args[0];
+    switch(ast_id(postfix))
+    {
+      case TK_NEWREF:
+      case TK_NEWBEREF:
+        r = args[0];
+        break;
+
+      default:
+        r = c->none_instance;
+        break;
+    }
   } else {
     while(arg != NULL)
     {

src/libponyc/codegen/genfun.c

@@ -359,8 +359,8 @@ static bool genfun_be(compile_t* c, reach_type_t* t, reach_method_t* m)
   // Send the arguments in a message to 'this'.
   gen_send_message(c, m, param_vals, params);
 
-  // Return 'this'.
-  LLVMBuildRet(c->builder, param_vals[0]);
+  // Return None.
+  LLVMBuildRet(c->builder, c->none_instance);
   codegen_finishfun(c);
 
   ponyint_pool_free_size(buf_size, param_vals);

src/libponyc/codegen/gentype.c

@@ -745,6 +745,12 @@ bool gentypes(compile_t* c)
     make_global_instance(c, t);
   }
 
+  // Cache the instance of None, which is used as the return value for
+  // behaviour calls.
+  t = reach_type_name(c->reach, "None");
+  assert(t != NULL);
+  c->none_instance = t->instance;
+
   if(c->opt->verbosity >= VERBOSITY_INFO)
     fprintf(stderr, " Function prototypes\n");
 

src/libponyc/pass/sugar.c

@@ -378,8 +378,9 @@ static ast_result_t sugar_be(pass_opt_t* opt, ast_t* ast)
 
   if(ast_id(result) == TK_NONE)
   {
-    // Return type is This tag
-    ast_replace(&result, type_for_this(opt, ast, TK_TAG, TK_NONE, false));
+    // Return type is None.
+    ast_t* type = type_sugar(ast, NULL, "None");
+    ast_replace(&result, type);
   }
 
   sugar_docstring(ast);

test/libponyc/sugar.cc

@@ -194,7 +194,7 @@ TEST_F(SugarTest, ActorWithCreateBehaviour)
   const char* full_form =
     "use \"builtin\"\n"
     "actor tag Foo\n"
-    "  be tag create():Foo tag => 3";
+    "  be tag create():None => 3";
 
   TEST_EQUIV(short_form, full_form);
 }
@@ -345,7 +345,7 @@ TEST_F(SugarTest, BehaviourReturnType)
     "use \"builtin\"\n"
     "actor tag Foo\n"
     "  var create: U32\n"
-    "  be tag foo():Foo tag => 3";
+    "  be tag foo():None => 3";
 
   TEST_EQUIV(short_form, full_form);
 }
@@ -1549,7 +1549,7 @@ TEST_F(SugarTest, ObjectWithBehaviour)
     "    None\n"
 
     "actor tag $T\n"
-    "  be tag foo(): $T tag =>\n"
+    "  be tag foo(): None =>\n"
     "    4\n"
     "  new tag create(): $T tag^ => true";
 
@@ -1626,7 +1626,7 @@ TEST_F(SugarTest, ObjectTagWithBehaviour)
     "    None\n"
 
     "actor tag $T\n"
-    "  be tag foo(): $T tag =>\n"
+    "  be tag foo(): None =>\n"
     "    4\n"
     "  new tag create(): $T tag^ => true";
 
@@ -2498,7 +2498,7 @@ TEST_F(SugarTest, CaseBehaviour)
     "use \"builtin\"\n"
     "actor tag Foo\n"
     "  var create: U32\n"
-    "  be tag fib(a: U64): Foo tag =>\n"
+    "  be tag fib(a: U64): None =>\n"
     "    $1(consume a)\n"
     "  fun ref $1($2: U64): None =>\n"
     "    match consume $2\n"

test/libponyc/sugar_expr.cc

@@ -214,7 +214,7 @@ TEST_F(SugarExprTest, PartialBehaviour)
 
     "class Foo\n"
     "  fun f(x: T) =>\n"
-    "    {box()($1 = x): T tag => $1.f() }";
+    "    {box()($1 = x) => $1.f() }";
 
   TEST_EQUIV(short_form, full_form);
 }

test/libponyc/type_check_subtype.cc

@@ -987,3 +987,49 @@ TEST_F(SubTypeTest, IsSubTypeArrow)
 
   pass_opt_init(&opt);
 }
+
+
+TEST_F(SubTypeTest, IsFunTagSubBe)
+{
+  const char* src =
+    "trait tag T\n"
+    "  be foo()\n"
+
+    "class tag C is T\n"
+    "  fun tag foo() => None\n"
+
+    "interface Test\n"
+    "  fun z(t: T, c: C)";
+
+  TEST_COMPILE(src);
+
+  pass_opt_t opt;
+  pass_opt_init(&opt);
+
+  ASSERT_TRUE(is_subtype(type_of("c"), type_of("t"), NULL, &opt));
+
+  pass_opt_init(&opt);
+}
+
+
+TEST_F(SubTypeTest, IsBeSubFunTag)
+{
+  const char* src =
+    "trait tag T\n"
+    "  fun tag foo()\n"
+
+    "actor A is T\n"
+    "  be foo() => None\n"
+
+    "interface Test\n"
+    "  fun z(t: T, a: A)";
+
+  TEST_COMPILE(src);
+
+  pass_opt_t opt;
+  pass_opt_init(&opt);
+
+  ASSERT_TRUE(is_subtype(type_of("a"), type_of("t"), NULL, &opt));
+
+  pass_opt_init(&opt);
+}

test/libponyc/verify.cc

@@ -186,10 +186,9 @@ TEST_F(VerifyTest, ActorBeFinal)
     "  be _final() =>\n"
     "    None";
 
-  TEST_ERRORS_3(src,
+  TEST_ERRORS_2(src,
     "a _final method must be a function",
-    "a _final method must use box as the receiver capability",
-    "a _final method must return None");
+    "a _final method must use box as the receiver capability");
 }
 
 TEST_F(VerifyTest, ClassFunRefFinal)