You can define an inline function by using
defun. An inline function works just like an ordinary
function except for one thing: when you compile a call to the function,
the function's definition is open-coded into the caller.
Making a function inline makes explicit calls run faster. But it also has disadvantages. For one thing, it reduces flexibility; if you change the definition of the function, calls already inlined still use the old definition until you recompile them.
Another disadvantage is that making a large function inline can increase the size of compiled code both in files and in memory. Since the speed advantage of inline functions is greatest for small functions, you generally should not make large functions inline.
Also, inline functions do not behave well with respect to debugging,
tracing, and advising (see Advising Functions). Since ease of
debugging and the flexibility of redefining functions are important
features of Emacs, you should not make a function inline, even if it's
small, unless its speed is really crucial, and you've timed the code
to verify that using
defun actually has performance problems.
It's possible to define a macro to expand into the same code that an
inline function would execute. (See Macros.) But the macro would be
limited to direct use in expressions—a macro cannot be called with
mapcar and so on. Also, it takes some work to
convert an ordinary function into a macro. To convert it into an inline
function is very easy; simply replace
Since each argument of an inline function is evaluated exactly once, you
needn't worry about how many times the body uses the arguments, as you
do for macros. (See Argument Evaluation.)
Inline functions can be used and open-coded later on in the same file, following the definition, just like macros.blog comments powered by Disqus