A lambda is simply a function.
Functions do not have to be named. Consider the fact that in English, “function” is also a noun. It is an ordinary value, just like numbers, lists, and characters.
Just like how a list can be created with (list 'item 'item 'item), a function can be created with (lambda (input) body-expr).
(lambda (x y) (+ x y)) produces a function that takes two arguments and sends them to the + function. In a typical environment:
((lambda (x y) (+ x y)) 2 3) ; => 5How to read this:
- The expression is a list of three values:
(lambda …),2, and3. - The first expression is evaluated first, resulting in a function value.
- The second and third values are evaluated in turn. Numbers evaluate to be themselves.
- The function value receives the second and third values as arguments and performs the calculation.
- The result of that calculation becomes the value of the whole expression after being evaluated.
Arguments
The way the argument list is given depends on the language. In Scheme-based languages, the first argument to lambda is:
- a list, each element in the list is bound to a local variable:
(lambda (x y) (+ x y)) - an “improper list” (like
(a b . c)) behaves like the above, but the last element is the rest argument, bound to a list containing arguments not bound by the positional arguments:(lambda (a b . c) (string-append a b (string-join c))) - a symbol; it becomes the rest argument:
(lambda input (string-join input))
Binding to an identifier
define is used to assign an identifier to a value: (define x 9) binds x to 9. In the same way, (define my-add (lambda (x y z) (+ x y z))) binds my-add to the value (lambda (x y z) (+ x y z)). Now my-add can be used like the lambda itself.
((lambda (x y z) (+ x y z)) 1 2 3)
; => 6
(define my-add
(lambda (x y z) (+ x y z)))
(my-add 1 2 3)
; => 6Shorthand for defining functions
define has a shorthand for creating functions:
(define my-add (lambda (x y) (+ x y)))can be written as:
(define (my-add x y) (+ x y))