Info Catalog eintr: Lisp Interpreter eintr: Byte Compiling

eintr: Complications

 
 Complications
 -------------
 
 Now, for the first complication.  In addition to lists, the Lisp
 interpreter can evaluate a symbol that is not quoted and does not have
 parentheses around it.  The Lisp interpreter will attempt to determine
 the symbol’s value as a “variable”.  This situation is described in the
 section on variables.  (Variables.)
 
    The second complication occurs because some functions are unusual and
 do not work in the usual manner.  Those that don’t are called “special
 forms”.  They are used for special jobs, like defining a function, and
 there are not many of them.  In the next few chapters, you will be
 introduced to several of the more important special forms.
 
    As well as special forms, there are also “macros”.  A macro is a
 construct defined in Lisp, which differs from a function in that it
 translates a Lisp expression into another expression that is to be
 evaluated in place of the original expression.  (Lisp macro.)
 
    For the purposes of this introduction, you do not need to worry too
 much about whether something is a special form, macro, or ordinary
 function.  For example, ‘if’ is a special form (if), but ‘when’
 is a macro (Lisp macro).  In earlier versions of Emacs, ‘defun’
 was a special form, but now it is a macro (defun).  It still
 behaves in the same way.
 
    The final complication is this: if the function that the Lisp
 interpreter is looking at is not a special form, and if it is part of a
 list, the Lisp interpreter looks to see whether the list has a list
 inside of it.  If there is an inner list, the Lisp interpreter first
 figures out what it should do with the inside list, and then it works on
 the outside list.  If there is yet another list embedded inside the
 inner list, it works on that one first, and so on.  It always works on
 the innermost list first.  The interpreter works on the innermost list
 first, to evaluate the result of that list.  The result may be used by
 the enclosing expression.
 
    Otherwise, the interpreter works left to right, from one expression
 to the next.
 

Info Catalog eintr: Lisp Interpreter eintr: Byte Compiling