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schemeracketmit-scheme
Maxfield
Maxfield

Reputation: 209

Scheme Lambda? What is it?

What is it in scheme? How can we use it ?

scm> (define (x) 100)
x
scm> (x)
100
scm> x ; When we "called" x, it return (lambda () 100). what is it ?
(lambda () 100)

Upvotes: 0

Views: 567

Answers (2)

Terence
Terence

Reputation: 1

(define (x) 100) is equivalent to (define x (lambda () 100)). The first syntax is actually a syntactic sugar of the second. Both define a procedure.

When you use x, the Scheme interpreter will return you x itself. In this case, x is a lambda expression. When you use (x), you are calling a procedure named x. The scheme interpreter will apply what's inside x, in this case, a lambda expression, and return you the return value.

To use lambda, consider lambda expression as an operator. So ((lambda (<arg-list>) <body>) <parameter-list>) Or, if you define a lambda expression to a variable, take the variable as the operator, for example, as what you do,

(define x
    (lambda () 100))
(x)

Upvotes: 0

Sylwester
Sylwester

Reputation: 48775

(define (x) 100) is the same as:

(define x          ; define the variable named x
        (lambda () ; as a anoymous function with zero arguments
          100))    ; that returns 100

x   ; ==> #<function> (some representation of the evaluated lambda object, there is no standard way)
(x) ; ==> 100 (The result of calling the function)

You might be more in to Algol languages so here is the same in JavaScript:

function x () { return 100; } is the same as:

var x =          // define the variable named x
  function () {  // as the anonymous function with zero arguments
    return 100;  // that returns 100
  };
x;   // => function () { return 100; } (prints its source)
x(); // => 100 (the result of calling the function)

Beginners sometimes add parentheses around variables like ((x)) and it is equivalent to writing x()() in Algol languages. Thus x must be a function of zero arguments that will return a function of zero arguments in order to work.

Upvotes: 5

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