#### 3.8.5 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.