Re: About `funcall'

["Pascal J. Bourguignon" <pjb@informatimago.com>]

Xue Fuqiao <xfq.free@gmail.com> writes:

> In `yank-pop', there is an sexp about `funcall':
>
>   (funcall (or yank-undo-function 'delete-region) (point) (mark t))
>
> IIRC the first argument for `funcall' should be a function, so I'm
> confused with this usage.  Can somebody explain it to me?  Thanks.

`or' returns the first argument that is not nil, or nil when all the
arguments are nil.

If the value bound to yank-undo-function is not nil, then it's returned,
else the symbol delete-region is returned (since it's not nil).

    (or a b) <=> (let ((first-arg a)) 
                   (if first-arg
                      first-arg
                      b))

The temporary variable is needed for (let ((i 0)) (or (incf i) 42))
returns 1, not 2.

    (defmacro .or (&rest args)
      (if (null args)
         'nil
         (let ((var (gensym)))
          `(let ((,var ,(first args)))
             (if ,var
                ,var
                (.or ,@(rest args)))))))

    (macroexpand '(.or a b))
    --> (let ((#1=#:G90861 a)) (if #1# #1# (\.or b)))



-- 
__Pascal Bourguignon__                     http://www.informatimago.com/
A bad day in () is better than a good day in {}.