reassign values

vm/vm.go

@@ -158,6 +158,17 @@ func (vm *VM) Run() error {
 
 			vm.globals[globalIndex] = vm.pop()
 
+		case code.OpAssign:
+			ident := vm.pop()
+			val := vm.pop()
+
+			obj, ok := ident.(object.Mutable)
+			if !ok {
+				return fmt.Errorf("cannot assign to %s", ident.Type())
+			}
+
+			obj.Set(val)
+
 		case code.OpGetGlobal:
 			globalIndex := code.ReadUint16(ins[ip+1:])
 			vm.currentFrame().ip += 2

vm/vm_test.go

@@ -801,10 +801,21 @@ func TestIterations(t *testing.T) {
 		{"while (false) { }", nil},
 		{"let n = 0; while (n < 10) { let n = n + 1 }; n", 10},
 		{"let n = 10; while (n > 0) { let n = n - 1 }; n", 0},
-		// FIXME: let is an expression statement and bind new values
-		//        there is currently no assignment expressions :/
-		{"let n = 0; while (n < 10) { let n = n + 1 }", nil},
-		{"let n = 10; while (n > 0) { let n = n - 1 }", nil},
+		{"let n = 0; while (n < 10) { n = n + 1 }; n", 10},
+		{"let n = 10; while (n > 0) { n = n - 1 }; n", 0},
+		{"let n = 0; while (n < 10) { n = n + 1 }", nil},
+		{"let n = 10; while (n > 0) { n = n - 1 }", nil},
+	}
+
+	runVmTests(t, tests)
+}
+
+func TestAssignmentStatements(t *testing.T) {
+	tests := []vmTestCase{
+		{"let one = 0; one = 1", 1},
+		{"let one = 0; one = 1; one", 1},
+		{"let one = 0; one = 1; let two = 0; two = 2; one + two", 3},
+		{"let one = 0; one = 1; let two = 0; two = one + one; one + two", 3},
 	}
 
 	runVmTests(t, tests)