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Diffstat (limited to 'lispref/eval.texi')
| -rw-r--r-- | lispref/eval.texi | 66 |
1 files changed, 32 insertions, 34 deletions
diff --git a/lispref/eval.texi b/lispref/eval.texi index 1bbd672f5fc..494a8145baf 100644 --- a/lispref/eval.texi +++ b/lispref/eval.texi @@ -73,6 +73,12 @@ of the form @code{(car x)}: the subform @code{x} must first be evaluated recursively, so that its value can be passed as an argument to the function @code{car}. + Evaluation of a function call ultimately calls the function specified +in it. @xref{Functions}. The execution of the function may itself work +by evaluating the function definition; or the function may be a Lisp +primitive implemented in C, or it may be a byte-compiled function +(@pxref{Byte Compilation}). + @cindex environment The evaluation of forms takes place in a context called the @dfn{environment}, which consists of the current values and bindings of @@ -90,17 +96,12 @@ is complete. The form may also make changes that persist; these changes are called @dfn{side effects}. An example of a form that produces side effects is @code{(setq foo 1)}. - Finally, evaluation of one particular function call, @code{byte-code}, -invokes the @dfn{byte-code interpreter} on its arguments. Although the -byte-code interpreter is not the same as the Lisp interpreter, it uses -the same environment as the Lisp interpreter, and may on occasion invoke -the Lisp interpreter. (@xref{Byte Compilation}.) - The details of what evaluation means for each kind of form are described below (@pxref{Forms}). @node Eval @section Eval +@c ??? Perhaps this should be the last section in the chapter. Most often, forms are evaluated automatically, by virtue of their occurrence in a program being run. On rare occasions, you may need to @@ -108,10 +109,14 @@ write code that evaluates a form that is computed at run time, such as after reading a form from text being edited or getting one from a property list. On these occasions, use the @code{eval} function. - The functions and variables described in this section evaluate -forms, specify limits to the evaluation process, or record recently -returned values. Loading a file also does evaluation -(@pxref{Loading}). + @strong{Note:} it is generally cleaner and more flexible to call +functions that are stored in data structures, rather than to evaluate +expressions stored in data structures. Using functions provides the +ability to pass information to them as arguments. + + The functions and variables described in this section evaluate forms, +specify limits to the evaluation process, or record recently returned +values. Loading a file also does evaluation (@pxref{Loading}). @defun eval form This is the basic function for performing evaluation. It evaluates @@ -143,19 +148,6 @@ The number of currently active calls to @code{eval} is limited to @code{max-lisp-eval-depth} (see below). @end defun -@cindex evaluation of buffer contents -@deffn Command eval-current-buffer &optional stream -This function evaluates the forms in the current buffer. It reads -forms from the buffer and calls @code{eval} on them until the end of the -buffer is reached, or until an error is signaled and not handled. - -If @var{stream} is supplied, the variable @code{standard-output} is -bound to @var{stream} during the evaluation (@pxref{Output -Functions}). - -@code{eval-current-buffer} always returns @code{nil}. -@end deffn - @deffn Command eval-region start end &optional stream This function evaluates the forms in the current buffer in the region defined by the positions @var{start} and @var{end}. It reads forms from @@ -172,6 +164,12 @@ expressions. @xref{How Programs Do Loading}. @code{eval-region} always returns @code{nil}. @end deffn +@cindex evaluation of buffer contents +@deffn Command eval-current-buffer &optional stream +This is like @code{eval-region} except that it operates on the whole +buffer. +@end deffn + @defvar max-lisp-eval-depth This variable defines the maximum depth allowed in calls to @code{eval}, @code{apply}, and @code{funcall} before an error is signaled (with error @@ -296,9 +294,9 @@ unchanged. It is common to write numbers, characters, strings, and even vectors in Lisp code, taking advantage of the fact that they self-evaluate. However, it is quite unusual to do this for types that lack a read -syntax, because there's no way to write them textually; however, it is -possible to construct Lisp expressions containing these types by means -of a Lisp program. Here is an example: +syntax, because there's no way to write them textually. It is possible +to construct Lisp expressions containing these types by means of a Lisp +program. Here is an example: @example @group @@ -654,14 +652,15 @@ definition loaded from that file. @xref{Autoload}. @section Quoting @cindex quoting - The special form @code{quote} returns its single argument -``unchanged''. + The special form @code{quote} returns its single argument, as written, +without evaluating it. This provides a way to include constant symbols +and lists, which are not self-evaluating objects, in a program. (It is +not necessary to quote self-evaluating objects such as numbers, strings, +and vectors.) @defspec quote object -This special form returns @var{object}, without evaluating it. This -provides a way to include constant symbols and lists, which are not -self-evaluating objects, in a program. (It is not necessary to quote -self-evaluating objects such as numbers, strings, and vectors.) +This special form returns @var{object}, without evaluating it. +@end defspec @cindex @samp{'} for quoting @cindex quoting using apostrophe @@ -700,9 +699,8 @@ Here are some examples of expressions that use @code{quote}: @result{} [(quote foo)] @end group @end example -@end defspec Other quoting constructs include @code{function} (@pxref{Anonymous Functions}), which causes an anonymous lambda expression written in Lisp -to be compiled, and @code{`} (@pxref{Backquote}), which is used to quote +to be compiled, and @samp{`} (@pxref{Backquote}), which is used to quote only part of a list, while computing and substituting other parts. |