Consecutive calls/evaluations in a form?

Question

Hey guys, simple question...

Working with XLISP to write a program, but I've seemed to run into a simple fundamental problem that I can't seem to work around: perhaps someone has a quick fix.

I'm trying to write an if statement who's then-clause evaluates multiple forms and returns the value of the last.

In example:

(setq POSITION 'DINING-ROOM)

(defun LOOK (DIRECTION ROOM) ... )
(defun SETPOS (ROOM) ... )
(defun WHERE () ... )

(defun MOVE (DIRECTION)
(if (not(equal nil (LOOK DIRECTION POSITION))) ; If there is a room in that direction
( ; Then-block: Go to that room. Return where you are.
(SETPOS (LOOK DIRECTION ROOM))
(WHERE)
)
( ; Else-block: Return error
(list 'CANT 'GO 'THERE)
)
)

The logical equivalent intended is:

function Move (Direction)
{

if(Look(Direction, Room) != null)
{
SetPos(Look(Direction,Room));
return Where();
}
else
{
return "Can't go there";
}

}

(Apologies for the poor web-formatting.)

The problem I have is with:

(
(SETPOS (LOOK DIRECTION ROOM))
(WHERE)
)

I simply want to return the evaluation of WHERE, but I need to execute the SETPOS function first. XLISP doesn't like the extra parentheses: if I remove the outer set, my WHERE list becomes my else (I don't want that). If I remove the sets around SETPOS and WHERE, it treats WHERE like an argument for SETPOS; I also don't want that.

So, how do I simply evaluate the first, then the second and then return the values of the last evaluated?

Answer 1

Lisp usually provides something like PROGN. PROGN evaluates a sequence of expressions and the value(s) of the last expression is returned.

(progn
  (do-this)
  (do-that))

Also look at your code:

(if (not(equal nil (LOOK DIRECTION POSITION)))

(EQUAL NIL (FOO))  is the same as  (NULL FOO)

(NOT (NULL FOO)) is the same as FOO.

So you can simply write:

(if (LOOK DIRECTION POSITION) ... ...)

Or if you want to check if there is a room:

(if (ROOM-P (LOOK DIRECTION POSITION)) ... ...)

ROOM-P would be a predicate that returns T if something is a room.

You may also want to use typical Lisp indentation:

(defun MOVE (DIRECTION)
  (if (LOOK DIRECTION POSITION)
    (progn
      (SETPOS (LOOK DIRECTION ROOM))
      (WHERE))
    (progn
      ...
      (list 'CANT 'GO 'THERE))))

There is also a COND construct:

(defun MOVE (DIRECTION)
  (cond ((LOOK DIRECTION POSITION)
         (SETPOS (LOOK DIRECTION ROOM))
         (WHERE))
        (t
         ...
         (list 'CANT 'GO 'THERE))))

I would also propose to switch from XLISP to something like CLISP or ECL. XLISP is old, mostly not maintained and not Common Lisp.

Answer 2

So I found a way of doing consecutive executions (whether its the best means or not):

Changed:

(
(SETPOS (LOOK DIRECTION ROOM))
(WHERE)
)

To:

(let ()
(SETPOS (LOOK DIRECTION ROOM))
(WHERE)
)

Which executed both forms and returned the output of the last.