Anonymous Functions =================== In Lisp, a function is a list that starts with `lambda', a byte-code function compiled from such a list, or alternatively a primitive subr-object; names are "extra". Although usually functions are defined with `defun' and given names at the same time, it is occasionally more concise to use an explicit lambda expression--an anonymous function. Such a list is valid wherever a function name is. Any method of creating such a list makes a valid function. Even this: (setq silly (append (lambda (x)) (list (list '+ (* 3 4) 'x)))) => (lambda (x) (+ 12 x)) This computes a list that looks like `(lambda (x) (+ 12 x))' and makes it the value (_not_ the function definition!) of `silly'. Here is how we might call this function: (funcall silly 1) => 13 (It does _not_ work to write `(silly 1)', because this function is not the _function definition_ of `silly'. We have not given `silly' any function definition, just a value as a variable.) Most of the time, anonymous functions are constants that appear in your program. For example, you might want to pass one as an argument to the function `mapcar', which applies any given function to each element of a list. Here we define a function `change-property' which uses a function as its third argument: (defun change-property (symbol prop function) (let ((value (get symbol prop))) (put symbol prop (funcall function value)))) Here we define a function that uses `change-property', passing it a function to double a number: (defun double-property (symbol prop) (change-property symbol prop (lambda (x) (* 2 x)))) In such cases, we usually use the special form `function' instead of simple quotation to quote the anonymous function, like this: (defun double-property (symbol prop) (change-property symbol prop (function (lambda (x) (* 2 x))))) Using `function' instead of `quote' makes a difference if you compile the function `double-property'. For example, if you compile the second definition of `double-property', the anonymous function is compiled as well. By contrast, if you compile the first definition which uses ordinary `quote', the argument passed to `change-property' is the precise list shown: (lambda (x) (* x 2)) The Lisp compiler cannot assume this list is a function, even though it looks like one, since it does not know what `change-property' will do with the list. Perhaps it will check whether the CAR of the third element is the symbol `*'! Using `function' tells the compiler it is safe to go ahead and compile the constant function. We sometimes write `function' instead of `quote' when quoting the name of a function, but this usage is just a sort of comment: (function SYMBOL) == (quote SYMBOL) == 'SYMBOL The read syntax `#'' is a short-hand for using `function'. For example, #'(lambda (x) (* x x)) is equivalent to (function (lambda (x) (* x x))) - Special Form: function function-object This special form returns FUNCTION-OBJECT without evaluating it. In this, it is equivalent to `quote'. However, it serves as a note to the Emacs Lisp compiler that FUNCTION-OBJECT is intended to be used only as a function, and therefore can safely be compiled. Contrast this with `quote', in Note: Quoting. See `documentation' in Note: Accessing Documentation, for a realistic example using `function' and an anonymous function.
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