Declarations - Common Lisp Extensions

7 Declarations

Common Lisp includes a complex and powerful “declaration” mechanism that allows you to give the compiler special hints about the types of data that will be stored in particular variables, and about the ways those variables and functions will be used. This package defines versions of all the Common Lisp declaration forms: declare, locally, proclaim, declaim, and the.

Most of the Common Lisp declarations are not currently useful in Emacs Lisp, as the byte-code system provides little opportunity to benefit from type information, and special declarations are redundant in a fully dynamically-scoped Lisp. A few declarations are meaningful when the optimizing byte compiler is being used, however. Under the earlier non-optimizing compiler, these declarations will effectively be ignored.

— Function: proclaim decl-spec

This function records a “global” declaration specified by decl-spec. Since proclaim is a function, decl-spec is evaluated and thus should normally be quoted.

— Special Form: declaim decl-specs...

This macro is like proclaim, except that it takes any number of decl-spec arguments, and the arguments are unevaluated and unquoted. The declaim macro also puts an (eval-when (compile load eval) ...) around the declarations so that they will be registered at compile-time as well as at run-time. (This is vital, since normally the declarations are meant to influence the way the compiler treats the rest of the file that contains the declaim form.)

— Special Form: declare decl-specs...

This macro is used to make declarations within functions and other code. Common Lisp allows declarations in various locations, generally at the beginning of any of the many “implicit progns” throughout Lisp syntax, such as function bodies, let bodies, etc. Currently the only declaration understood by declare is special.

— Special Form: locally declarations... forms...

In this package, locally is no different from progn.

— Special Form: the type form

Type information provided by the is ignored in this package; in other words, (the type form) is equivalent to form. Future versions of the optimizing byte-compiler may make use of this information.

For example, mapcar can map over both lists and arrays. It is hard for the compiler to expand mapcar into an in-line loop unless it knows whether the sequence will be a list or an array ahead of time. With (mapcar 'car (the vector foo)), a future compiler would have enough information to expand the loop in-line. For now, Emacs Lisp will treat the above code as exactly equivalent to (mapcar 'car foo).

Each decl-spec in a proclaim, declaim, or declare should be a list beginning with a symbol that says what kind of declaration it is. This package currently understands special, inline, notinline, optimize, and warn declarations. (The warn declaration is an extension of standard Common Lisp.) Other Common Lisp declarations, such as type and ftype, are silently ignored.

special

Since all variables in Emacs Lisp are “special” (in the Common Lisp sense), special declarations are only advisory. They simply tell the optimizing byte compiler that the specified variables are intentionally being referred to without being bound in the body of the function. The compiler normally emits warnings for such references, since they could be typographical errors for references to local variables.

The declaration (declare (special var1 var2)) is equivalent to (defvar var1) (defvar var2) in the optimizing compiler, or to nothing at all in older compilers (which do not warn for non-local references).

In top-level contexts, it is generally better to write (defvar var) than (declaim (special var)), since defvar makes your intentions clearer. But the older byte compilers can not handle defvars appearing inside of functions, while (declare (special var)) takes care to work correctly with all compilers.

inline

The inline decl-spec lists one or more functions whose bodies should be expanded “in-line” into calling functions whenever the compiler is able to arrange for it. For example, the Common Lisp function cadr is declared inline by this package so that the form (cadr x) will expand directly into (car (cdr x)) when it is called in user functions, for a savings of one (relatively expensive) function call.

The following declarations are all equivalent. Note that the defsubst form is a convenient way to define a function and declare it inline all at once.

          (declaim (inline foo bar))
          (eval-when (compile load eval) (proclaim '(inline foo bar)))
          (defsubst foo (...) ...)       ; instead of defun

Please note: this declaration remains in effect after the containing source file is done. It is correct to use it to request that a function you have defined should be inlined, but it is impolite to use it to request inlining of an external function.

In Common Lisp, it is possible to use (declare (inline ...)) before a particular call to a function to cause just that call to be inlined; the current byte compilers provide no way to implement this, so (declare (inline ...)) is currently ignored by this package.

notinline

The notinline declaration lists functions which should not be inlined after all; it cancels a previous inline declaration.

optimize

This declaration controls how much optimization is performed by the compiler. Naturally, it is ignored by the earlier non-optimizing compilers.

The word optimize is followed by any number of lists like (speed 3) or (safety 2). Common Lisp defines several optimization “qualities”; this package ignores all but speed and safety. The value of a quality should be an integer from 0 to 3, with 0 meaning “unimportant” and 3 meaning “very important.” The default level for both qualities is 1.

In this package, with the optimizing compiler, the speed quality is tied to the byte-compile-optimize flag, which is set to nil for (speed 0) and to t for higher settings; and the safety quality is tied to the byte-compile-delete-errors flag, which is set to t for (safety 3) and to nil for all lower settings. (The latter flag controls whether the compiler is allowed to optimize out code whose only side-effect could be to signal an error, e.g., rewriting (progn foo bar) to bar when it is not known whether foo will be bound at run-time.)

Note that even compiling with (safety 0), the Emacs byte-code system provides sufficient checking to prevent real harm from being done. For example, barring serious bugs in Emacs itself, Emacs will not crash with a segmentation fault just because of an error in a fully-optimized Lisp program.

The optimize declaration is normally used in a top-level proclaim or declaim in a file; Common Lisp allows it to be used with declare to set the level of optimization locally for a given form, but this will not work correctly with the current version of the optimizing compiler. (The declare will set the new optimization level, but that level will not automatically be unset after the enclosing form is done.)

warn

This declaration controls what sorts of warnings are generated by the byte compiler. Again, only the optimizing compiler generates warnings. The word warn is followed by any number of “warning qualities,” similar in form to optimization qualities. The currently supported warning types are redefine, callargs, unresolved, and free-vars; in the current system, a value of 0 will disable these warnings and any higher value will enable them. See the documentation for the optimizing byte compiler for details.