Appendix A Efficiency Concerns

A.1 Macros

Many of the advanced features of this package, such as cl-defun, cl-loop, etc., are implemented as Lisp macros. In byte-compiled code, these complex notations will be expanded into equivalent Lisp code which is simple and efficient. For example, the form

(cl-incf i n)

is expanded at compile-time to the Lisp form

(setq i (+ i n))

which is the most efficient ways of doing this operation in Lisp. Thus, there is no performance penalty for using the more readable cl-incf form in your compiled code.

Interpreted code, on the other hand, must expand these macros every time they are executed. For this reason it is strongly recommended that code making heavy use of macros be compiled. A loop using cl-incf a hundred times will execute considerably faster if compiled, and will also garbage-collect less because the macro expansion will not have to be generated, used, and thrown away a hundred times.

You can find out how a macro expands by using the cl-prettyexpand function.

Function: cl-prettyexpand form &optional full

This function takes a single Lisp form as an argument and inserts a nicely formatted copy of it in the current buffer (which must be in Lisp mode so that indentation works properly). It also expands all Lisp macros that appear in the form. The easiest way to use this function is to go to the *scratch* buffer and type, say,

(cl-prettyexpand '(cl-loop for x below 10 collect x))

and type C-x C-e immediately after the closing parenthesis; an expansion similar to:

(cl-block nil
     (let* ((x 0)
            (G1004 nil))
       (while (< x 10)
         (setq G1004 (cons x G1004))
         (setq x (+ x 1)))
       (nreverse G1004)))

will be inserted into the buffer. (The cl-block macro is expanded differently in the interpreter and compiler, so cl-prettyexpand just leaves it alone. The temporary variable G1004 was created by cl-gensym.)

If the optional argument full is true, then all macros are expanded, including cl-block, cl-eval-when, and compiler macros. Expansion is done as if form were a top-level form in a file being compiled.

Note that cl-adjoin, cl-caddr, and cl-member all have built-in compiler macros to optimize them in common cases.

A.2 Error Checking

Common Lisp compliance has in general not been sacrificed for the sake of efficiency. A few exceptions have been made for cases where substantial gains were possible at the expense of marginal incompatibility.

The Common Lisp standard (as embodied in Steele’s book) uses the phrase “it is an error if” to indicate a situation that is not supposed to arise in complying programs; implementations are strongly encouraged but not required to signal an error in these situations. This package sometimes omits such error checking in the interest of compactness and efficiency. For example, cl-do variable specifiers are supposed to be lists of one, two, or three forms; extra forms are ignored by this package rather than signaling a syntax error. Functions taking keyword arguments will accept an odd number of arguments, treating the trailing keyword as if it were followed by the value nil.

Argument lists (as processed by cl-defun and friends) are checked rigorously except for the minor point just mentioned; in particular, keyword arguments are checked for validity, and &allow-other-keys and :allow-other-keys are fully implemented. Keyword validity checking is slightly time consuming (though not too bad in byte-compiled code); you can use &allow-other-keys to omit this check. Functions defined in this package such as cl-find and cl-member do check their keyword arguments for validity.

A.3 Compiler Optimizations

Changing the value of byte-optimize from the default t is highly discouraged; many of the Common Lisp macros emit code that can be improved by optimization. In particular, cl-blocks (whether explicit or implicit in constructs like cl-defun and cl-loop) carry a fair run-time penalty; the byte-compiler removes cl-blocks that are not actually referenced by cl-return or cl-return-from inside the block.