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  ANSI Common Lisp   3 Evaluation and Compilation   3.8 Dictionary Evaluation and Compilation

3.8.5 eval-when Special Operator

Syntax:
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 Section 3.2.3 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 Section 3.2 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, Section 3.2 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.

Allegro CL Implementation Details:
None.

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