how to add python-style decorator to my language

python-deco.md

how to add python-style decorator to my language

I followed https://craftinginterpreters.com/functions.html and function call is implemented.

The interpreter can interpret block like this:

fun deco(func) {
  fun impl() {
    print "start" + func;
    func();
    print "end" + func;
  }
  return impl;
}

fun hello(){print "hello";}
var h2 = deco(hello);
h2();

with the result:

start<fn hello>
hello
end<fn hello>

However, as I recalled Python decorator pattern, which is more concise, I want to add @ as a syntax sugar.

So I did the following modifications (github commit)

--- a/src/main/java/com/craftinginterpreters/lox/Scanner.java
+++ b/src/main/java/com/craftinginterpreters/lox/Scanner.java
@@ -31,6 +31,7 @@ public class Scanner {
         keywords.put("true", TRUE);
         keywords.put("var", VAR);
         keywords.put("while", WHILE);
+        keywords.put("@", DECO);
     }
 
     Scanner(String source) {
@@ -65,6 +66,7 @@ public class Scanner {
             case '+' -> addToken(PLUS);
             case ';' -> addToken(SEMICOLON);
             case '*' -> addToken(STAR);
+            case '@' -> addToken(DECO);
             case '!' -> addToken(match('=') ? BANG_EQUAL : BANG);
             case '=' -> addToken(match('=') ? EQUAL_EQUAL : EQUAL);
             case '<' -> addToken(match('=') ? LESS_EQUAL : LESS);

--- a/src/main/java/com/craftinginterpreters/lox/Parser.java
+++ b/src/main/java/com/craftinginterpreters/lox/Parser.java
@@ -25,6 +25,7 @@ public class Parser {
         try {
             if (match(FUN)) return function("function");
             if (match(VAR)) return varDeclaration();
+            if (match(DECO)) return decoDecl();
             return statement();
         } catch (ParseError error) {
             synchonize();
@@ -32,19 +33,32 @@ public class Parser {
         }
     }
 
+    private Stmt decoDecl() {
+        Token deco_method = consume(IDENTIFIER, "expect deco name");
+        consume(FUN, "expect function after decorator");
+        Stmt.Function e2 = function("function");
+        Expr e3 = new Expr.Call(
+            new Expr.Variable(deco_method),
+             null, 
+             Arrays.asList(
+                new Expr.Variable(e2.name)
+            )
+        );
+        return new Stmt.Var(e2.name, e3);
+    }
+

However, I don't know how to put the previous function declarations.

So it only works like this:

fun deco(func) {
  fun impl() {
    print "start" + func;
    func();
    print "end" + func;
  }
  return impl;
}


fun hello(){print "hello";}  
^--- If removed this line the interpreter cannot found variable `hello`

@deco
fun hello(){print "hello";}

hello();

I tried using Stmt.Block but since it creates a new scope the function declaration is discarded after exited the scope.

Any Suggestions?