Question

I am VERY new to scheme. Very basic question for everyone. What is lambda in function calls I see everywhere online? What does it do? What do I lose by not using it?

Answer 1

lambda is a special form in the family of Lisp programming langauges (and in other languages with some degree of support for functional programming), it’s an anonymous function that creates a closure around the context in which it was defined.

Lambdas are used everywhere in Lisp, either explicitly or implicitly (behind macro transformations, implicitly in other special forms, etc.) What would you lose by not using it? Everything. For starters, the ability to define procedures!

To see some examples of what I mean, this procedure definition:

(define (f x)
  x)

… Is just syntactic sugar for this, a named procedure that simply associates a name to the anonymous lambda:

(define f
  (lambda (x)
    x))

Similarly, the let form that binds values to variables:

(let ((x 9))
  (+ x 1))

… Is just syntactic sugar for this:

((lambda (x)
   (+ x 1))
 9)

Of course, lambdas can be used as a quick way to define anonymous functions:

(map (lambda (x) (* x x))
     '(1 2 3 4 5))

Truth be told, it’s almost impossible not to use lambdas for writing any non-trivial program in Lisp.