13.14 Inline Functions

An inline function is a function that works just like an ordinary function, except for one thing: when you byte-compile a call to the function (see Byte Compilation), the function’s definition is expanded into the caller.

The simple way to define an inline function, is to write defsubst instead of defun. The rest of the definition looks just the same, but using defsubst says to make it inline for byte compilation.

Macro: defsubst name args [doc] [declare] [interactive] body…

This macro defines an inline function. Its syntax is exactly the same as defun (see Defining Functions).

Making a function inline often makes its function 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 Emacs Lisp 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.

After an inline function is defined, its inline expansion can be performed later on in the same file, just like macros.

It’s possible to use defmacro 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 apply, 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 easy; just replace defun with defsubst. 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.

Alternatively, you can define a function by providing the code which will inline it as a compiler macro (see The declare Form). The following macros make this possible.

Macro: define-inline name args [doc] [declare] body…

Define a function name by providing code that does its inlining, as a compiler macro. The function will accept the argument list args and will have the specified body.

If present, doc should be the function’s documentation string (see Documentation Strings of Functions); declare, if present, should be a declare form (see The declare Form) specifying the function’s metadata.

Functions defined via define-inline have several advantages with respect to macros defined by defsubst or defmacro:

Like defmacro, a function inlined with define-inline inherits the scoping rules, either dynamic or lexical, from the call site. See Scoping Rules for Variable Bindings.

The following macros should be used in the body of a function defined by define-inline.

Macro: inline-quote expression

Quote expression for define-inline. This is similar to the backquote (see Backquote), but quotes code and accepts only ,, not ,@.

Macro: inline-letevals (bindings…) body…

This provides a convenient way to ensure that the arguments to an inlined function are evaluated exactly once, as well as to create local variables.

It’s similar to let (see Local Variables): It sets up local variables as specified by bindings, and then evaluates body with those bindings in effect.

Each element of bindings should be either a symbol or a list of the form (var expr); the result is to evaluate expr and bind var to the result. However, when an element of bindings is just a symbol var, the result of evaluating var is re-bound to var (which is quite different from the way let works).

The tail of bindings can be either nil or a symbol which should hold a list of arguments, in which case each argument is evaluated, and the symbol is bound to the resulting list.

Macro: inline-const-p expression

Return non-nil if the value of expression is already known.

Macro: inline-const-val expression

Return the value of expression.

Macro: inline-error format &rest args

Signal an error, formatting args according to format.

Here’s an example of using define-inline:

(define-inline myaccessor (obj)
  (inline-letevals (obj)
    (inline-quote (if (foo-p ,obj) (aref (cdr ,obj) 3) (aref ,obj 2)))))

This is equivalent to

(defsubst myaccessor (obj)
  (if (foo-p obj) (aref (cdr obj) 3) (aref obj 2)))