Info Catalog eintr: insert-buffer body eintr: insert-buffer eintr: Insert or

eintr: if & or

 
 5.2.3 ‘insert-buffer’ With an ‘if’ Instead of an ‘or’
 -----------------------------------------------------
 
 The job to be done is to make sure the value of ‘buffer’ is a buffer
 itself and not the name of a buffer.  If the value is the name, then the
 buffer itself must be got.
 
    You can imagine yourself at a conference where an usher is wandering
 around holding a list with your name on it and looking for you: the
 usher is bound to your name, not to you; but when the usher finds you
 and takes your arm, the usher becomes bound to you.
 
    In Lisp, you might describe this situation like this:
 
      (if (not (holding-on-to-guest))
          (find-and-take-arm-of-guest))
 
    We want to do the same thing with a buffer—if we do not have the
 buffer itself, we want to get it.
 
    Using a predicate called ‘bufferp’ that tells us whether we have a
 buffer (rather than its name), we can write the code like this:
 
      (if (not (bufferp buffer))              ; if-part
          (setq buffer (get-buffer buffer)))  ; then-part
 
 Here, the true-or-false-test of the ‘if’ expression is
 ‘(not (bufferp buffer))’; and the then-part is the expression
 ‘(setq buffer (get-buffer buffer))’.
 
    In the test, the function ‘bufferp’ returns true if its argument is a
 buffer—but false if its argument is the name of the buffer.  (The last
 character of the function name ‘bufferp’ is the character ‘p’; as we saw
 earlier, such use of ‘p’ is a convention that indicates that the
 function is a predicate, which is a term that means that the function
 will determine whether some property is true or false.  Using the
 Wrong Type Object as an Argument Wrong Type of Argument.)
 
    The function ‘not’ precedes the expression ‘(bufferp buffer)’, so the
 true-or-false-test looks like this:
 
      (not (bufferp buffer))
 
 ‘not’ is a function that returns true if its argument is false and false
 if its argument is true.  So if ‘(bufferp buffer)’ returns true, the
 ‘not’ expression returns false and vice versa.
 
    Using this test, the ‘if’ expression works as follows: when the value
 of the variable ‘buffer’ is actually a buffer rather than its name, the
 true-or-false-test returns false and the ‘if’ expression does not
 evaluate the then-part.  This is fine, since we do not need to do
 anything to the variable ‘buffer’ if it really is a buffer.
 
    On the other hand, when the value of ‘buffer’ is not a buffer itself,
 but the name of a buffer, the true-or-false-test returns true and the
 then-part of the expression is evaluated.  In this case, the then-part
 is ‘(setq buffer (get-buffer buffer))’.  This expression uses the
 ‘get-buffer’ function to return an actual buffer itself, given its name.
 The ‘setq’ then sets the variable ‘buffer’ to the value of the buffer
 itself, replacing its previous value (which was the name of the buffer).
 

Info Catalog eintr: insert-buffer body eintr: insert-buffer eintr: Insert or