Declarations, especially type and function declarations, increase the efficiency of the compiled code. For example, for the following Lisp source file, with two Common-Lisp declarations added,

   (eval-when (compile)
   (proclaim '(function tak (fixnum fixnum fixnum) fixnum))

   (defun tak (x y z)
   (declare (fixnum x y z))
   (if (not (< y x))
   z
   (tak (tak (1- x) y z)
   (tak (1- y) z x)
   (tak (1- z) x y))))
  

The compiler generates the following C code:

   /*      local entry for function TAK                                  */
   static int LI1(register int V1,register int V2,register int V3)
   { VT3 VLEX3 CLSR3
   TTL:
   if (V2 < V1) {
   goto L2;}
   return(V3);
   L2:
   { int V5;
   V5 = LI1((V1)-1,V2,V3);
   { int V6;
   V6 = LI1((V2)-1,V3,V1);
   V3 = LI1((V3)-1,V1,V2);
   V2 = V6;
   V1 = V5;}}
   goto TTL;
   ;;; Note: Tail-recursive call of TAK was replaced by iteration.
   }