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(cl)Declarations
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 `progn's"
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
`defvar's 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
*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.
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