Improve REPL support

This fixes a bug with expressions inside loops in interactive mode
that resulted in expressions not being printed:

    >> for i in range(3):
    ...    i
    ...
    >>

Python prints the value every iteration:

    >> for i in range(3):
    ...    i
    ...
    0
    1
    2
    >>>

The same also applies to while loops.

src/codegen/cpython_ast.cpp

@@ -61,6 +61,8 @@ private:
     InternedStringPool* pool = NULL;
     int loop_depth = 0;
     int in_finally = 0;
+    int interactive = 0;
+    int nestlevel = 0;
     llvm::StringRef fn;
 
 public:
@@ -511,7 +513,9 @@ public:
                 auto v = stmt->v.FunctionDef;
                 r->name = convert(v.name);
                 r->args = convert(v.args, r);
+                nestlevel++;
                 r->body = convert<stmt_ty, AST_stmt*>(v.body);
+                nestlevel--;
                 r->decorator_list = convert<expr_ty, AST_expr*>(v.decorator_list);
                 return r;
             }
@@ -520,7 +524,9 @@ public:
                 auto v = stmt->v.ClassDef;
                 r->name = convert(v.name);
                 r->bases = convert<expr_ty, AST_expr*>(v.bases);
+                nestlevel++;
                 r->body = convert<stmt_ty, AST_stmt*>(v.body);
+                nestlevel--;
                 r->decorator_list = convert<expr_ty, AST_expr*>(v.decorator_list);
                 return r;
             }
@@ -666,6 +672,11 @@ public:
                 auto r = new AST_Expr();
                 auto v = stmt->v.Expr;
                 r->value = convert(v.value);
+                if (interactive && nestlevel <= 0) {
+                    auto print_expr = new AST_LangPrimitive(AST_LangPrimitive::PRINT_EXPR);
+                    print_expr->args.push_back(r->value);
+                    r->value = print_expr;
+                }
                 return r;
             }
             case Pass_kind:
@@ -704,11 +715,11 @@ public:
                 return rtn;
             }
             case Interactive_kind: {
+                this->interactive = 1;
                 AST_Module* rtn = new AST_Module(llvm::make_unique<InternedStringPool>());
                 assert(!this->pool);
                 this->pool = rtn->interned_strings.get();
                 convertAll<stmt_ty>(mod->v.Interactive.body, rtn->body);
-                makeModuleInteractive(rtn);
                 return rtn;
             }
             case Expression_kind: {

src/core/ast.cpp

@@ -2236,17 +2236,4 @@ void flatten(AST_expr* root, std::vector<AST*>& output, bool expand_scopes) {
 
     root->accept(&visitor);
 }
-
-void makeModuleInteractive(AST_Module* m) {
-    for (int i = 0; i < m->body.size(); ++i) {
-        AST_stmt* s = m->body[i];
-        if (s->type != AST_TYPE::Expr)
-            continue;
-
-        AST_Expr* expr = (AST_Expr*)s;
-        AST_LangPrimitive* print_expr = new AST_LangPrimitive(AST_LangPrimitive::PRINT_EXPR);
-        print_expr->args.push_back(expr->value);
-        expr->value = print_expr;
-    }
-}
 }

src/core/ast.h

@@ -1444,10 +1444,6 @@ template <class T, class R> void findNodes(const R& roots, std::vector<T*>& outp
     }
 }
 
-// Take a normally-parsed module, and convert it (inplace) to a form that will print out any bare expressions.
-// This is used for "single" mode or the repl.
-void makeModuleInteractive(AST_Module* m);
-
 llvm::StringRef getOpSymbol(int op_type);
 BORROWED(BoxedString*) getOpName(int op_type);
 int getReverseCmpOp(int op_type, bool& success);