Next: Defining Named Functions Up: Program Structure Previous: Lambda-Expressions
Common Lisp the Language, 2nd Edition
Next: Defining Named
Functions Up: Program
Structure Previous: Lambda-Expressions
The standard way for the user to interact with a Common Lisp
implementation is via a read-eval-print loop: the system
repeatedly reads a form from some input source (such as a keyboard or a
disk file), evaluates it, and then prints the value(s) to some output
sink (such as a display screen or another disk file). Any form
(evaluable data object) is acceptable; however, certain special forms
are specifically designed to be convenient for use as top-level
forms, rather than as forms embedded within other forms in the way that
(+ 3 4) is embedded within (if p (+ 3 4) 6).
These top-level special forms may be used to define globally named
functions, to define macros, to make declarations, and to define global
values for special variables.
It is not illegal to use these forms at other than top level, but
whether it is meaningful to do so depends on context. Compilers, for
example, may not recognize these forms properly in other than top-level
contexts. (As a special case, however, if a progn form
appears at top level, then all forms within that progn are
considered by the compiler to be top-level forms.)
X3J13 voted in March 1989 (DEFINING-MACROS-NON-TOP-LEVEL) to clarify
that, while defining forms normally appear at top level, it is
meaningful to place them in non-top-level contexts. All defining forms
that create functional objects from code appearing as argument forms
must ensure that such argument forms refer to the enclosing lexical
environment. Compilers must handle defining forms properly in all
situations, not just top-level contexts. However, certain compile-time
side effects of these defining forms are performed only when the
defining forms occur at top level (see section 25.1).
Compatibility note: In MacLisp, a top-level
progn is considered to contain top-level forms only if the
first form is (quote compile). This odd marker is
unnecessary in Common Lisp.
Macros are usually defined by using the special form
defmacro. This facility is fairly complicated; it is
described in chapter 8.
Next: Defining Named
Functions Up: Program
Structure Previous: Lambda-Expressions
AI.Repository@cs.cmu.edu