Worksheet L-Vals

Lox does not have a static type checker. Write a program that is accepted by lox, but would be rejected if lox had static types like Java.

Java has static types, but also a full dynamic type system. Take the Java program that you wrote above and convert it to a program that runs in Java. Hint: Declare all variables at type Object.

In static scope, a variable occurrence is always bound to the same declaration/environment.

In dynamic scope, a variable occurrence may be bound to different declarations/environments.

Explore this concept in Javascript with statements by executing the following code in an online REPL or on your machine using your browser or Node.js:

var scope = {};

function f () {
    with (scope) {
        console.log (x);
    };    
}

// f (); // ReferenceError: x is not defined

var x = 1;
f ();    // 1

scope.x = "dog"
f();     // dog

Lox only allows assignment to simple names. Other languages use the concept of l-value to distinguish assignable expressions.

In each of the following contexts, determine whether the given expression is an l-value or not. Your code should replace the ellipsis (...).

1. Is x++ an l-value in C? The context is:

   void f (int x) {
     ...
   }
   

2. Is arr[x++] an l-value in C? The context is:

   void f (int x) {
     int arr[] = { 5, 6, 7, 8, 9, 10, 11 };
     ...
   }
   

3. Is f ().x an l-value in C? The context is:

   struct S {
     int x;
     int y;
   };
   
   struct S f () {
     struct S s;
     s.x = 5;
     s.y = 6;
     return s;  // returns a copy of the "struct S", i.e., copies the two int members back
   }
   
   void g () {
     ...
   } 
   

4. Is t.x an l-value in C? The context is:

   struct S {
     int x;
     int y;
   };
   
   struct S f () {
     struct S s;
     s.x = 5;
     s.y = 6;
     return s;  // returns a copy of the "struct S", i.e., copies the two int members back
   }
   
   void g () {
     struct S t = f ();
     ...
   } 
   

5. Is f ()->x an l-value in C? Recall that this means (*(f())).x, i.e., call f, dereference the pointer, then access the x member of the struct. The context is:

   #include <stdlib.h>
   
   struct S {
     int x;
     int y;
   };
   
   struct S *f () {
     struct S *p = (struct S *) malloc (sizeof (struct S));
     p->x = 5;   // recall that p->x is just shorthand for (*p).x
     p->y = 6;
     return p;  // returns a copy of the pointer, i.e., copies just a pointer back
   }
   
   void g () {
     ...
   } 
   

6. Is arr[x++] an l-value in Java? The context is:

   class LValue6 {
     static void f (int x) {
       int[] arr = { 5, 6, 7, 8, 9, 10, 11 };
       ...
     }
   }
   

7. Is list.get (x++) an l-value in Java? The context is:

   import java.util.ArrayList;
   
   class LValue7 {
     static void f (int x) {
       ArrayList<Integer> list = new ArrayList<> (); 
       list.add (5); 
       list.add (6); 
       list.add (7); 
       list.add (8); 
       ...
     }
   }
   

8. Is list.get (x++).x an l-value in Java? The context is:

   import java.util.ArrayList;
   
   class C {
     int x;
     int y;
     C (int x, int y) {
       this.x = x;
       this.y = y;
     }
   }
   
   class LValue8 {
     static void f (int x) {
       ArrayList<C> list = new ArrayList<> (); 
       list.add (new C (5, 5)); 
       list.add (new C (6, 6)); 
       list.add (new C (7, 7)); 
       list.add (new C (8, 8)); 
       ...
     }
   }
   

Solution: L-Value or Not?