1994-03-28 20:21:44 +00:00
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@c -*-texinfo-*-
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@c This is part of the GNU Emacs Lisp Reference Manual.
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@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1998 Free Software Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
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@setfilename ../info/eval
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@node Evaluation, Control Structures, Symbols, Top
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@chapter Evaluation
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@cindex evaluation
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@cindex interpreter
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@cindex interpreter
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@cindex value of expression
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The @dfn{evaluation} of expressions in Emacs Lisp is performed by the
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@dfn{Lisp interpreter}---a program that receives a Lisp object as input
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and computes its @dfn{value as an expression}. How it does this depends
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on the data type of the object, according to rules described in this
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chapter. The interpreter runs automatically to evaluate portions of
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your program, but can also be called explicitly via the Lisp primitive
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function @code{eval}.
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@ifnottex
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@menu
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* Intro Eval:: Evaluation in the scheme of things.
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* Forms:: How various sorts of objects are evaluated.
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* Quoting:: Avoiding evaluation (to put constants in the program).
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* Eval:: How to invoke the Lisp interpreter explicitly.
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@end menu
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@node Intro Eval
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@section Introduction to Evaluation
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1994-04-26 22:21:24 +00:00
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The Lisp interpreter, or evaluator, is the program that computes
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the value of an expression that is given to it. When a function
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written in Lisp is called, the evaluator computes the value of the
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function by evaluating the expressions in the function body. Thus,
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running any Lisp program really means running the Lisp interpreter.
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How the evaluator handles an object depends primarily on the data
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type of the object.
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@end ifnottex
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@cindex forms
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@cindex expression
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A Lisp object that is intended for evaluation is called an
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@dfn{expression} or a @dfn{form}. The fact that expressions are data
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objects and not merely text is one of the fundamental differences
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between Lisp-like languages and typical programming languages. Any
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object can be evaluated, but in practice only numbers, symbols, lists
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and strings are evaluated very often.
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It is very common to read a Lisp expression and then evaluate the
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expression, but reading and evaluation are separate activities, and
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either can be performed alone. Reading per se does not evaluate
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anything; it converts the printed representation of a Lisp object to the
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object itself. It is up to the caller of @code{read} whether this
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object is a form to be evaluated, or serves some entirely different
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purpose. @xref{Input Functions}.
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Do not confuse evaluation with command key interpretation. The
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editor command loop translates keyboard input into a command (an
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interactively callable function) using the active keymaps, and then
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uses @code{call-interactively} to invoke the command. The execution of
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the command itself involves evaluation if the command is written in
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Lisp, but that is not a part of command key interpretation itself.
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@xref{Command Loop}.
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@cindex recursive evaluation
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Evaluation is a recursive process. That is, evaluation of a form may
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call @code{eval} to evaluate parts of the form. For example, evaluation
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of a function call first evaluates each argument of the function call,
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and then evaluates each form in the function body. Consider evaluation
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of the form @code{(car x)}: the subform @code{x} must first be evaluated
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recursively, so that its value can be passed as an argument to the
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function @code{car}.
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Evaluation of a function call ultimately calls the function specified
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in it. @xref{Functions}. The execution of the function may itself work
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by evaluating the function definition; or the function may be a Lisp
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primitive implemented in C, or it may be a byte-compiled function
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(@pxref{Byte Compilation}).
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@cindex environment
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The evaluation of forms takes place in a context called the
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@dfn{environment}, which consists of the current values and bindings of
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all Lisp variables.@footnote{This definition of ``environment'' is
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specifically not intended to include all the data that can affect the
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result of a program.} Whenever a form refers to a variable without
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creating a new binding for it, the value of the variable's binding in
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the current environment is used. @xref{Variables}.
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@cindex side effect
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Evaluation of a form may create new environments for recursive
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evaluation by binding variables (@pxref{Local Variables}). These
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environments are temporary and vanish by the time evaluation of the form
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is complete. The form may also make changes that persist; these changes
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are called @dfn{side effects}. An example of a form that produces side
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effects is @code{(setq foo 1)}.
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The details of what evaluation means for each kind of form are
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described below (@pxref{Forms}).
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@node Forms
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@section Kinds of Forms
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A Lisp object that is intended to be evaluated is called a @dfn{form}.
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How Emacs evaluates a form depends on its data type. Emacs has three
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different kinds of form that are evaluated differently: symbols, lists,
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and ``all other types''. This section describes all three kinds, one by
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one, starting with the ``all other types'' which are self-evaluating
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forms.
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@menu
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* Self-Evaluating Forms:: Forms that evaluate to themselves.
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* Symbol Forms:: Symbols evaluate as variables.
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* Classifying Lists:: How to distinguish various sorts of list forms.
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* Function Indirection:: When a symbol appears as the car of a list,
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we find the real function via the symbol.
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* Function Forms:: Forms that call functions.
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* Macro Forms:: Forms that call macros.
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* Special Forms:: ``Special forms'' are idiosyncratic primitives,
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most of them extremely important.
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* Autoloading:: Functions set up to load files
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containing their real definitions.
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@end menu
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@node Self-Evaluating Forms
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@subsection Self-Evaluating Forms
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@cindex vector evaluation
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@cindex literal evaluation
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@cindex self-evaluating form
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A @dfn{self-evaluating form} is any form that is not a list or symbol.
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Self-evaluating forms evaluate to themselves: the result of evaluation
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is the same object that was evaluated. Thus, the number 25 evaluates to
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25, and the string @code{"foo"} evaluates to the string @code{"foo"}.
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Likewise, evaluation of a vector does not cause evaluation of the
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elements of the vector---it returns the same vector with its contents
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unchanged.
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@example
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@group
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'123 ; @r{A number, shown without evaluation.}
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@result{} 123
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@end group
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@group
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123 ; @r{Evaluated as usual---result is the same.}
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@result{} 123
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@end group
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@group
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(eval '123) ; @r{Evaluated ``by hand''---result is the same.}
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@result{} 123
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@end group
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@group
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(eval (eval '123)) ; @r{Evaluating twice changes nothing.}
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@result{} 123
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@end group
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@end example
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It is common to write numbers, characters, strings, and even vectors
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in Lisp code, taking advantage of the fact that they self-evaluate.
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However, it is quite unusual to do this for types that lack a read
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syntax, because there's no way to write them textually. It is possible
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to construct Lisp expressions containing these types by means of a Lisp
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program. Here is an example:
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@example
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@group
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;; @r{Build an expression containing a buffer object.}
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(setq print-exp (list 'print (current-buffer)))
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@result{} (print #<buffer eval.texi>)
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@end group
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@group
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;; @r{Evaluate it.}
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(eval print-exp)
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@print{} #<buffer eval.texi>
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@result{} #<buffer eval.texi>
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@end group
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@end example
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@node Symbol Forms
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@subsection Symbol Forms
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@cindex symbol evaluation
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When a symbol is evaluated, it is treated as a variable. The result
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is the variable's value, if it has one. If it has none (if its value
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cell is void), an error is signaled. For more information on the use of
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variables, see @ref{Variables}.
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In the following example, we set the value of a symbol with
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@code{setq}. Then we evaluate the symbol, and get back the value that
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@code{setq} stored.
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@example
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@group
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(setq a 123)
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@result{} 123
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@end group
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@group
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(eval 'a)
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@result{} 123
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@end group
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@group
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a
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@result{} 123
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@end group
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@end example
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The symbols @code{nil} and @code{t} are treated specially, so that the
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value of @code{nil} is always @code{nil}, and the value of @code{t} is
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always @code{t}; you cannot set or bind them to any other values. Thus,
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these two symbols act like self-evaluating forms, even though
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@code{eval} treats them like any other symbol. A symbol whose name
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starts with @samp{:} also self-evaluates in the same way; likewise,
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its value ordinarily cannot be changed. @xref{Constant Variables}.
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@node Classifying Lists
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@subsection Classification of List Forms
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@cindex list form evaluation
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A form that is a nonempty list is either a function call, a macro
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call, or a special form, according to its first element. These three
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kinds of forms are evaluated in different ways, described below. The
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remaining list elements constitute the @dfn{arguments} for the function,
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macro, or special form.
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The first step in evaluating a nonempty list is to examine its first
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element. This element alone determines what kind of form the list is
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and how the rest of the list is to be processed. The first element is
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@emph{not} evaluated, as it would be in some Lisp dialects such as
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Scheme.
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@node Function Indirection
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@subsection Symbol Function Indirection
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@cindex symbol function indirection
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@cindex indirection
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@cindex void function
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If the first element of the list is a symbol then evaluation examines
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the symbol's function cell, and uses its contents instead of the
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original symbol. If the contents are another symbol, this process,
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called @dfn{symbol function indirection}, is repeated until it obtains a
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non-symbol. @xref{Function Names}, for more information about using a
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symbol as a name for a function stored in the function cell of the
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symbol.
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One possible consequence of this process is an infinite loop, in the
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event that a symbol's function cell refers to the same symbol. Or a
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symbol may have a void function cell, in which case the subroutine
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@code{symbol-function} signals a @code{void-function} error. But if
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neither of these things happens, we eventually obtain a non-symbol,
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which ought to be a function or other suitable object.
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@kindex invalid-function
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@cindex invalid function
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More precisely, we should now have a Lisp function (a lambda
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expression), a byte-code function, a primitive function, a Lisp macro, a
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special form, or an autoload object. Each of these types is a case
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described in one of the following sections. If the object is not one of
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these types, the error @code{invalid-function} is signaled.
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The following example illustrates the symbol indirection process. We
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use @code{fset} to set the function cell of a symbol and
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@code{symbol-function} to get the function cell contents
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(@pxref{Function Cells}). Specifically, we store the symbol @code{car}
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into the function cell of @code{first}, and the symbol @code{first} into
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the function cell of @code{erste}.
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@smallexample
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@group
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;; @r{Build this function cell linkage:}
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;; ------------- ----- ------- -------
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;; | #<subr car> | <-- | car | <-- | first | <-- | erste |
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;; ------------- ----- ------- -------
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@end group
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@end smallexample
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@smallexample
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@group
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(symbol-function 'car)
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@result{} #<subr car>
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@end group
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@group
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(fset 'first 'car)
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@result{} car
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@end group
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@group
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(fset 'erste 'first)
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@result{} first
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@end group
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@group
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(erste '(1 2 3)) ; @r{Call the function referenced by @code{erste}.}
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@result{} 1
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@end group
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@end smallexample
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By contrast, the following example calls a function without any symbol
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function indirection, because the first element is an anonymous Lisp
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function, not a symbol.
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@smallexample
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@group
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((lambda (arg) (erste arg))
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'(1 2 3))
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@result{} 1
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@end group
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@end smallexample
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@noindent
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Executing the function itself evaluates its body; this does involve
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symbol function indirection when calling @code{erste}.
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The built-in function @code{indirect-function} provides an easy way to
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perform symbol function indirection explicitly.
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@c Emacs 19 feature
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@defun indirect-function function
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This function returns the meaning of @var{function} as a function. If
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@var{function} is a symbol, then it finds @var{function}'s function
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definition and starts over with that value. If @var{function} is not a
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symbol, then it returns @var{function} itself.
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Here is how you could define @code{indirect-function} in Lisp:
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@smallexample
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(defun indirect-function (function)
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(if (symbolp function)
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(indirect-function (symbol-function function))
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function))
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@end smallexample
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@end defun
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@node Function Forms
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@subsection Evaluation of Function Forms
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@cindex function form evaluation
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@cindex function call
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If the first element of a list being evaluated is a Lisp function
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object, byte-code object or primitive function object, then that list is
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a @dfn{function call}. For example, here is a call to the function
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@code{+}:
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|
|
|
|
|
|
@example
|
|
|
|
(+ 1 x)
|
|
|
|
@end example
|
|
|
|
|
1994-04-26 22:21:24 +00:00
|
|
|
The first step in evaluating a function call is to evaluate the
|
|
|
|
remaining elements of the list from left to right. The results are the
|
|
|
|
actual argument values, one value for each list element. The next step
|
|
|
|
is to call the function with this list of arguments, effectively using
|
|
|
|
the function @code{apply} (@pxref{Calling Functions}). If the function
|
|
|
|
is written in Lisp, the arguments are used to bind the argument
|
|
|
|
variables of the function (@pxref{Lambda Expressions}); then the forms
|
|
|
|
in the function body are evaluated in order, and the value of the last
|
|
|
|
body form becomes the value of the function call.
|
1994-03-28 20:21:44 +00:00
|
|
|
|
|
|
|
@node Macro Forms
|
|
|
|
@subsection Lisp Macro Evaluation
|
|
|
|
@cindex macro call evaluation
|
|
|
|
|
|
|
|
If the first element of a list being evaluated is a macro object, then
|
|
|
|
the list is a @dfn{macro call}. When a macro call is evaluated, the
|
|
|
|
elements of the rest of the list are @emph{not} initially evaluated.
|
|
|
|
Instead, these elements themselves are used as the arguments of the
|
|
|
|
macro. The macro definition computes a replacement form, called the
|
|
|
|
@dfn{expansion} of the macro, to be evaluated in place of the original
|
|
|
|
form. The expansion may be any sort of form: a self-evaluating
|
1994-04-26 22:21:24 +00:00
|
|
|
constant, a symbol, or a list. If the expansion is itself a macro call,
|
1994-03-28 20:21:44 +00:00
|
|
|
this process of expansion repeats until some other sort of form results.
|
|
|
|
|
1994-04-26 22:21:24 +00:00
|
|
|
Ordinary evaluation of a macro call finishes by evaluating the
|
|
|
|
expansion. However, the macro expansion is not necessarily evaluated
|
|
|
|
right away, or at all, because other programs also expand macro calls,
|
|
|
|
and they may or may not evaluate the expansions.
|
|
|
|
|
1994-03-28 20:21:44 +00:00
|
|
|
Normally, the argument expressions are not evaluated as part of
|
|
|
|
computing the macro expansion, but instead appear as part of the
|
1998-02-28 01:53:53 +00:00
|
|
|
expansion, so they are computed when the expansion is evaluated.
|
1994-03-28 20:21:44 +00:00
|
|
|
|
|
|
|
For example, given a macro defined as follows:
|
|
|
|
|
|
|
|
@example
|
|
|
|
@group
|
|
|
|
(defmacro cadr (x)
|
|
|
|
(list 'car (list 'cdr x)))
|
|
|
|
@end group
|
|
|
|
@end example
|
|
|
|
|
|
|
|
@noindent
|
|
|
|
an expression such as @code{(cadr (assq 'handler list))} is a macro
|
|
|
|
call, and its expansion is:
|
|
|
|
|
|
|
|
@example
|
|
|
|
(car (cdr (assq 'handler list)))
|
|
|
|
@end example
|
|
|
|
|
|
|
|
@noindent
|
|
|
|
Note that the argument @code{(assq 'handler list)} appears in the
|
|
|
|
expansion.
|
|
|
|
|
|
|
|
@xref{Macros}, for a complete description of Emacs Lisp macros.
|
|
|
|
|
|
|
|
@node Special Forms
|
|
|
|
@subsection Special Forms
|
|
|
|
@cindex special form evaluation
|
|
|
|
|
|
|
|
A @dfn{special form} is a primitive function specially marked so that
|
|
|
|
its arguments are not all evaluated. Most special forms define control
|
|
|
|
structures or perform variable bindings---things which functions cannot
|
|
|
|
do.
|
|
|
|
|
|
|
|
Each special form has its own rules for which arguments are evaluated
|
|
|
|
and which are used without evaluation. Whether a particular argument is
|
|
|
|
evaluated may depend on the results of evaluating other arguments.
|
|
|
|
|
|
|
|
Here is a list, in alphabetical order, of all of the special forms in
|
|
|
|
Emacs Lisp with a reference to where each is described.
|
|
|
|
|
|
|
|
@table @code
|
|
|
|
@item and
|
|
|
|
@pxref{Combining Conditions}
|
|
|
|
|
|
|
|
@item catch
|
|
|
|
@pxref{Catch and Throw}
|
|
|
|
|
|
|
|
@item cond
|
|
|
|
@pxref{Conditionals}
|
|
|
|
|
|
|
|
@item condition-case
|
|
|
|
@pxref{Handling Errors}
|
|
|
|
|
|
|
|
@item defconst
|
|
|
|
@pxref{Defining Variables}
|
|
|
|
|
|
|
|
@item defmacro
|
|
|
|
@pxref{Defining Macros}
|
|
|
|
|
|
|
|
@item defun
|
|
|
|
@pxref{Defining Functions}
|
|
|
|
|
|
|
|
@item defvar
|
|
|
|
@pxref{Defining Variables}
|
|
|
|
|
|
|
|
@item function
|
|
|
|
@pxref{Anonymous Functions}
|
|
|
|
|
|
|
|
@item if
|
|
|
|
@pxref{Conditionals}
|
|
|
|
|
|
|
|
@item interactive
|
|
|
|
@pxref{Interactive Call}
|
|
|
|
|
|
|
|
@item let
|
|
|
|
@itemx let*
|
|
|
|
@pxref{Local Variables}
|
|
|
|
|
|
|
|
@item or
|
|
|
|
@pxref{Combining Conditions}
|
|
|
|
|
|
|
|
@item prog1
|
|
|
|
@itemx prog2
|
|
|
|
@itemx progn
|
|
|
|
@pxref{Sequencing}
|
|
|
|
|
|
|
|
@item quote
|
|
|
|
@pxref{Quoting}
|
|
|
|
|
1998-05-19 03:45:57 +00:00
|
|
|
@item save-current-buffer
|
|
|
|
@pxref{Current Buffer}
|
|
|
|
|
1994-03-28 20:21:44 +00:00
|
|
|
@item save-excursion
|
|
|
|
@pxref{Excursions}
|
|
|
|
|
|
|
|
@item save-restriction
|
|
|
|
@pxref{Narrowing}
|
|
|
|
|
|
|
|
@item save-window-excursion
|
|
|
|
@pxref{Window Configurations}
|
|
|
|
|
|
|
|
@item setq
|
|
|
|
@pxref{Setting Variables}
|
|
|
|
|
|
|
|
@item setq-default
|
|
|
|
@pxref{Creating Buffer-Local}
|
|
|
|
|
|
|
|
@item track-mouse
|
|
|
|
@pxref{Mouse Tracking}
|
|
|
|
|
|
|
|
@item unwind-protect
|
|
|
|
@pxref{Nonlocal Exits}
|
|
|
|
|
|
|
|
@item while
|
|
|
|
@pxref{Iteration}
|
|
|
|
|
|
|
|
@item with-output-to-temp-buffer
|
|
|
|
@pxref{Temporary Displays}
|
|
|
|
@end table
|
|
|
|
|
|
|
|
@cindex CL note---special forms compared
|
|
|
|
@quotation
|
1994-04-26 22:21:24 +00:00
|
|
|
@b{Common Lisp note:} Here are some comparisons of special forms in
|
1994-03-28 20:21:44 +00:00
|
|
|
GNU Emacs Lisp and Common Lisp. @code{setq}, @code{if}, and
|
|
|
|
@code{catch} are special forms in both Emacs Lisp and Common Lisp.
|
|
|
|
@code{defun} is a special form in Emacs Lisp, but a macro in Common
|
|
|
|
Lisp. @code{save-excursion} is a special form in Emacs Lisp, but
|
|
|
|
doesn't exist in Common Lisp. @code{throw} is a special form in
|
|
|
|
Common Lisp (because it must be able to throw multiple values), but it
|
|
|
|
is a function in Emacs Lisp (which doesn't have multiple
|
|
|
|
values).@refill
|
|
|
|
@end quotation
|
|
|
|
|
|
|
|
@node Autoloading
|
|
|
|
@subsection Autoloading
|
|
|
|
|
|
|
|
The @dfn{autoload} feature allows you to call a function or macro
|
|
|
|
whose function definition has not yet been loaded into Emacs. It
|
|
|
|
specifies which file contains the definition. When an autoload object
|
|
|
|
appears as a symbol's function definition, calling that symbol as a
|
|
|
|
function automatically loads the specified file; then it calls the real
|
|
|
|
definition loaded from that file. @xref{Autoload}.
|
|
|
|
|
|
|
|
@node Quoting
|
|
|
|
@section Quoting
|
|
|
|
@cindex quoting
|
|
|
|
|
1995-06-06 19:21:15 +00:00
|
|
|
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.)
|
1994-03-28 20:21:44 +00:00
|
|
|
|
|
|
|
@defspec quote object
|
1995-06-06 19:21:15 +00:00
|
|
|
This special form returns @var{object}, without evaluating it.
|
|
|
|
@end defspec
|
1994-03-28 20:21:44 +00:00
|
|
|
|
|
|
|
@cindex @samp{'} for quoting
|
|
|
|
@cindex quoting using apostrophe
|
|
|
|
@cindex apostrophe for quoting
|
|
|
|
Because @code{quote} is used so often in programs, Lisp provides a
|
|
|
|
convenient read syntax for it. An apostrophe character (@samp{'})
|
|
|
|
followed by a Lisp object (in read syntax) expands to a list whose first
|
|
|
|
element is @code{quote}, and whose second element is the object. Thus,
|
|
|
|
the read syntax @code{'x} is an abbreviation for @code{(quote x)}.
|
|
|
|
|
|
|
|
Here are some examples of expressions that use @code{quote}:
|
|
|
|
|
|
|
|
@example
|
|
|
|
@group
|
|
|
|
(quote (+ 1 2))
|
|
|
|
@result{} (+ 1 2)
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(quote foo)
|
|
|
|
@result{} foo
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
'foo
|
|
|
|
@result{} foo
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
''foo
|
|
|
|
@result{} (quote foo)
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
'(quote foo)
|
|
|
|
@result{} (quote foo)
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
['foo]
|
|
|
|
@result{} [(quote foo)]
|
|
|
|
@end group
|
|
|
|
@end example
|
|
|
|
|
|
|
|
Other quoting constructs include @code{function} (@pxref{Anonymous
|
|
|
|
Functions}), which causes an anonymous lambda expression written in Lisp
|
1995-06-06 19:21:15 +00:00
|
|
|
to be compiled, and @samp{`} (@pxref{Backquote}), which is used to quote
|
1994-03-28 20:21:44 +00:00
|
|
|
only part of a list, while computing and substituting other parts.
|
1998-02-28 01:53:53 +00:00
|
|
|
|
|
|
|
@node Eval
|
|
|
|
@section Eval
|
|
|
|
|
|
|
|
Most often, forms are evaluated automatically, by virtue of their
|
|
|
|
occurrence in a program being run. On rare occasions, you may need to
|
|
|
|
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 store a
|
|
|
|
function in a data structure, and call it with @code{funcall} or
|
|
|
|
@code{apply}, than to store an expression in the data structure and
|
|
|
|
evaluate it. Using functions provides the ability to pass information
|
|
|
|
to them as arguments.
|
|
|
|
|
|
|
|
@defun eval form
|
|
|
|
This is the basic function evaluating an expression. It evaluates
|
|
|
|
@var{form} in the current environment and returns the result. How the
|
|
|
|
evaluation proceeds depends on the type of the object (@pxref{Forms}).
|
|
|
|
|
|
|
|
Since @code{eval} is a function, the argument expression that appears
|
|
|
|
in a call to @code{eval} is evaluated twice: once as preparation before
|
|
|
|
@code{eval} is called, and again by the @code{eval} function itself.
|
|
|
|
Here is an example:
|
|
|
|
|
|
|
|
@example
|
|
|
|
@group
|
|
|
|
(setq foo 'bar)
|
|
|
|
@result{} bar
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(setq bar 'baz)
|
|
|
|
@result{} baz
|
|
|
|
;; @r{Here @code{eval} receives argument @code{foo}}
|
|
|
|
(eval 'foo)
|
|
|
|
@result{} bar
|
|
|
|
;; @r{Here @code{eval} receives argument @code{bar}, which is the value of @code{foo}}
|
|
|
|
(eval foo)
|
|
|
|
@result{} baz
|
|
|
|
@end group
|
|
|
|
@end example
|
|
|
|
|
|
|
|
The number of currently active calls to @code{eval} is limited to
|
|
|
|
@code{max-lisp-eval-depth} (see below).
|
|
|
|
@end defun
|
|
|
|
|
1998-06-10 04:08:49 +00:00
|
|
|
@deffn Command eval-region start end &optional stream read-function
|
1998-02-28 01:53:53 +00:00
|
|
|
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
|
|
|
|
the region and calls @code{eval} on them until the end of the region is
|
|
|
|
reached, or until an error is signaled and not handled.
|
|
|
|
|
1998-06-10 04:08:49 +00:00
|
|
|
If @var{stream} is non-@code{nil}, the values that result from
|
|
|
|
evaluating the expressions in the region are printed using @var{stream}.
|
|
|
|
@xref{Output Streams}.
|
1998-02-28 01:53:53 +00:00
|
|
|
|
1998-06-10 04:08:49 +00:00
|
|
|
If @var{read-function} is non-@code{nil}, it should be a function, which
|
|
|
|
is used instead of @code{read} to read expressions one by one. This
|
|
|
|
function is called with one argument, the stream for reading input. You
|
|
|
|
can also use the variable @code{load-read-function} (@pxref{How Programs
|
|
|
|
Do Loading}) to specify this function, but it is more robust to use the
|
|
|
|
@var{read-function} argument.
|
1998-02-28 01:53:53 +00:00
|
|
|
|
|
|
|
@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
|
1998-04-20 17:43:57 +00:00
|
|
|
message @code{"Lisp nesting exceeds max-lisp-eval-depth"}). This limit,
|
|
|
|
with the associated error when it is exceeded, is one way that Lisp
|
|
|
|
avoids infinite recursion on an ill-defined function.
|
1998-02-28 01:53:53 +00:00
|
|
|
@cindex Lisp nesting error
|
|
|
|
|
1998-04-20 17:43:57 +00:00
|
|
|
The depth limit counts internal uses of @code{eval}, @code{apply}, and
|
|
|
|
@code{funcall}, such as for calling the functions mentioned in Lisp
|
|
|
|
expressions, and recursive evaluation of function call arguments and
|
|
|
|
function body forms, as well as explicit calls in Lisp code.
|
|
|
|
|
1998-05-19 03:45:57 +00:00
|
|
|
The default value of this variable is 300. If you set it to a value
|
1998-02-28 01:53:53 +00:00
|
|
|
less than 100, Lisp will reset it to 100 if the given value is reached.
|
1998-05-19 03:45:57 +00:00
|
|
|
Entry to the Lisp debugger increases the value, if there is little room
|
|
|
|
left, to make sure the debugger itself has room to execute.
|
1998-02-28 01:53:53 +00:00
|
|
|
|
|
|
|
@code{max-specpdl-size} provides another limit on nesting.
|
|
|
|
@xref{Local Variables}.
|
|
|
|
@end defvar
|
|
|
|
|
|
|
|
@defvar values
|
|
|
|
The value of this variable is a list of the values returned by all the
|
|
|
|
expressions that were read, evaluated, and printed from buffers
|
|
|
|
(including the minibuffer) by the standard Emacs commands which do this.
|
|
|
|
The elements are ordered most recent first.
|
|
|
|
|
|
|
|
@example
|
|
|
|
@group
|
|
|
|
(setq x 1)
|
|
|
|
@result{} 1
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
(list 'A (1+ 2) auto-save-default)
|
|
|
|
@result{} (A 3 t)
|
|
|
|
@end group
|
|
|
|
@group
|
|
|
|
values
|
|
|
|
@result{} ((A 3 t) 1 @dots{})
|
|
|
|
@end group
|
|
|
|
@end example
|
|
|
|
|
|
|
|
This variable is useful for referring back to values of forms recently
|
|
|
|
evaluated. It is generally a bad idea to print the value of
|
|
|
|
@code{values} itself, since this may be very long. Instead, examine
|
|
|
|
particular elements, like this:
|
|
|
|
|
|
|
|
@example
|
|
|
|
@group
|
|
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;; @r{Refer to the most recent evaluation result.}
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(nth 0 values)
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@result{} (A 3 t)
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@end group
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@group
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;; @r{That put a new element on,}
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;; @r{so all elements move back one.}
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(nth 1 values)
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@result{} (A 3 t)
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@end group
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@group
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;; @r{This gets the element that was next-to-most-recent}
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;; @r{before this example.}
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(nth 3 values)
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@result{} 1
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@end group
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@end example
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@end defvar
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