(var x 5)                       // Lan 7 or 8
(var x true)                    // Lan 7 or 8
(var x (lambda w (* w w)))      // Lan 8 (anonymous lanmbda expression)

(apply f 5)                     // Lan 7 or 8
(apply (lambda w (+ x x)) 5)    // Lang 8 (anonymous lanmbda expression)

(fun f (lambda x (* x x)))            // Lan 7 or 8
(fun f (lambda x (+ x x)))            // Lan 7 or 8
(fun f (lambda x (apply g x)))        // Lan 7 or 8
(fun f (lambda x (lambda y (+ y x)))  // Lan 8 (anonymous lanmbda expression)

f(x)  // higher order function
{
   return y => y + x;  // nested function return value
}

g = f(5);
g(6);     ==> 11
f(5)(6);  ==> 11


f(x, g)  // higher order function
{
   return x + g(x);  // function input parameter
}

(fun f (lambda x g (+ x (apply g x))))  // Lan 7 or 8
(fun sq (lambda x (* x x)))             // Lan 7 or 8
(apply f 5 sq) ==> 30                   // Lan 7 or 8
(apply f 5 (lambda y (* y y))) ==> 30   // Lan 8 only


Two kinds of higer order functions,
    function parameters   (Lan 7 or 8),
    function return value (Lan 7 or 8).
In Language 8 both kinds of higher order functions can use anonymous lanmbda expressions.

(fun h (lambda f g x (if (< x 0) f g))) // Lan 7 or 8


Lan 7
  higher order functions
  lambda expressions
Lan 8
  anonymous lambda expressions
  nested lambda expressions
  closures
  garbage collection