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Programming in Emacs Lisp

C.4.3 A lambda Expression: Useful Anonymity

lambda is the symbol for an anonymous function, a function without a name. Every time you use an anonymous function, you need to include its whole body.

Thus,

(lambda (arg) (/ arg 50))

is a function definition that says `return the value resulting from dividing whatever is passed to me as arg by 50'.

Earlier, for example, we had a function multiply-by-seven; it multiplied its argument by 7. This function is similar, except it divides its argument by 50; and, it has no name. The anonymous equivalent of multiply-by-seven is:

(lambda (number) (* 7 number))

(See section The defun Special Form.)

If we want to multiply 3 by 7, we can write:

(multiply-by-seven 3) \_______________/ ^ | | function argument

This expression returns 21.

Similarly, we can write:

((lambda (number) (* 7 number)) 3) \____________________________/ ^ | | anonymous function argument

If we want to divide 100 by 50, we can write:

((lambda (arg) (/ arg 50)) 100) \______________________/ \_/ | | anonymous function argument

This expression returns 2. The 100 is passed to the function, which divides that number by 50.

See section `Lambda Expressions' in The GNU Emacs Lisp Reference Manual, for more about lambda. Lisp and lambda expressions derive from the Lambda Calculus.