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eval-when [Special Operator]

eval-when ({situation}*) {form}* => {result}*

Arguments and Values::

situation--One of the symbols :compile-toplevel , :load-toplevel , :execute , compile , load , or eval .

The use of eval, compile, and load is deprecated.

forms--an implicit progn.

results--the values of the forms if they are executed, or nil if they are not.

Description::

The body of an eval-when form is processed as an implicit progn, but only in the situations listed.

The use of the situations :compile-toplevel (or compile) and :load-toplevel (or load) controls whether and when evaluation occurs when eval-when appears as a top level form in code processed by compile-file. See File Compilation.

The use of the situation :execute (or eval) controls whether evaluation occurs for other eval-when forms; that is, those that are not top level forms, or those in code processed by eval or compile. If the :execute situation is specified in such a form, then the body forms are processed as an implicit progn; otherwise, the eval-when form returns nil.

eval-when normally appears as a top level form, but it is meaningful for it to appear as a non-top-level form. However, the compile-time side effects described in Compilation only take place when eval-when appears as a top level form.

Examples::

One example of the use of eval-when is that for the compiler to be able to read a file properly when it uses user-defined reader macros, it is necessary to write

      (eval-when (:compile-toplevel :load-toplevel :execute)
        (set-macro-character #\$ #'(lambda (stream char)
                                     (declare (ignore char))
                                     (list 'dollar (read stream))))) =>  T
     

This causes the call to set-macro-character to be executed in the compiler's execution environment, thereby modifying its reader syntax table.

     ;;;     The EVAL-WHEN in this case is not at toplevel, so only the :EXECUTE
     ;;;     keyword is considered. At compile time, this has no effect.
     ;;;     At load time (if the LET is at toplevel), or at execution time
     ;;;     (if the LET is embedded in some other form which does not execute
     ;;;     until later) this sets (SYMBOL-FUNCTION 'FOO1) to a function which
     ;;;     returns 1.
      (let ((x 1))
        (eval-when (:execute :load-toplevel :compile-toplevel)
          (setf (symbol-function 'foo1) #'(lambda () x))))
     
     ;;;     If this expression occurs at the toplevel of a file to be compiled,
     ;;;     it has BOTH a compile time AND a load-time effect of setting
     ;;;     (SYMBOL-FUNCTION 'FOO2) to a function which returns 2.
      (eval-when (:execute :load-toplevel :compile-toplevel)
        (let ((x 2))
          (eval-when (:execute :load-toplevel :compile-toplevel)
            (setf (symbol-function 'foo2) #'(lambda () x)))))
     
     ;;;     If this expression occurs at the toplevel of a file to be compiled,
     ;;;     it has BOTH a compile time AND a load-time effect of setting the
     ;;;     function cell of FOO3 to a function which returns 3.
      (eval-when (:execute :load-toplevel :compile-toplevel)
        (setf (symbol-function 'foo3) #'(lambda () 3)))
     
     ;;; #4: This always does nothing. It simply returns NIL.
      (eval-when (:compile-toplevel)
        (eval-when (:compile-toplevel)
          (print 'foo4)))
     
     ;;;     If this form occurs at toplevel of a file to be compiled, FOO5 is
     ;;;     printed at compile time. If this form occurs in a non-top-level
     ;;;     position, nothing is printed at compile time. Regardless of context,
     ;;;     nothing is ever printed at load time or execution time.
      (eval-when (:compile-toplevel)
        (eval-when (:execute)
          (print 'foo5)))
     
     ;;;     If this form occurs at toplevel of a file to be compiled, FOO6 is
     ;;;     printed at compile time.  If this form occurs in a non-top-level
     ;;;     position, nothing is printed at compile time. Regardless of context,
     ;;;     nothing is ever printed at load time or execution time.
      (eval-when (:execute :load-toplevel)
        (eval-when (:compile-toplevel)
          (print 'foo6)))
     

See Also::

compile-file , Compilation

Notes::

The following effects are logical consequences of the definition of eval-when:


*
Execution of a single eval-when expression executes the body code at most once.
*
Macros intended for use in top level forms should be written so that side-effects are done by the forms in the macro expansion. The macro-expander itself should not do the side-effects.

For example:

Wrong:

           (defmacro foo ()
             (really-foo)
             `(really-foo))
          

Right:

           (defmacro foo ()
             `(eval-when (:compile-toplevel :execute :load-toplevel) (really-foo)))
          

Adherence to this convention means that such macros behave intuitively when appearing as non-top-level forms.

*
Placing a variable binding around an eval-when reliably captures the binding because the compile-time-too mode cannot occur (i.e., introducing a variable binding means that the eval-when is not a top level form). For example,
           (let ((x 3))
             (eval-when (:execute :load-toplevel :compile-toplevel) (print x)))
          

prints 3 at execution (i.e., load) time, and does not print anything at compile time. This is important so that expansions of defun and defmacro can be done in terms of eval-when and can correctly capture the lexical environment.

           (defun bar (x) (defun foo () (+ x 3)))
          

might expand into

           (defun bar (x)
             (progn (eval-when (:compile-toplevel)
                      (compiler::notice-function-definition 'foo '(x)))
                    (eval-when (:execute :load-toplevel)
                      (setf (symbol-function 'foo) #'(lambda () (+ x 3))))))
          

which would be treated by the above rules the same as

           (defun bar (x)
             (setf (symbol-function 'foo) #'(lambda () (+ x 3))))
          

when the definition of bar is not a top level form.