Info Catalog elisp: Argument Evaluation elisp: Problems with Macros elisp: Eval During Expansion

elisp: Surprising Local Vars

 
 13.5.3 Local Variables in Macro Expansions
 ------------------------------------------
 
 In the previous section, the definition of ‘for’ was fixed as follows to
 make the expansion evaluate the macro arguments the proper number of
 times:
 
      (defmacro for (var from init to final do &rest body)
        "Execute a simple for loop: (for i from 1 to 10 do (print i))."
        `(let ((,var ,init)
               (max ,final))
           (while (<= ,var max)
             ,@body
             (inc ,var))))
 
    The new definition of ‘for’ has a new problem: it introduces a local
 variable named ‘max’ which the user does not expect.  This causes
 trouble in examples such as the following:
 
      (let ((max 0))
        (for x from 0 to 10 do
          (let ((this (frob x)))
            (if (< max this)
                (setq max this)))))
 
 The references to ‘max’ inside the body of the ‘for’, which are supposed
 to refer to the user’s binding of ‘max’, really access the binding made
 by ‘for’.
 
    The way to correct this is to use an uninterned symbol instead of
 ‘max’ (Creating Symbols).  The uninterned symbol can be bound
 and referred to just like any other symbol, but since it is created by
 ‘for’, we know that it cannot already appear in the user’s program.
 Since it is not interned, there is no way the user can put it into the
 program later.  It will never appear anywhere except where put by ‘for’.
 Here is a definition of ‘for’ that works this way:
 
      (defmacro for (var from init to final do &rest body)
        "Execute a simple for loop: (for i from 1 to 10 do (print i))."
        (let ((tempvar (make-symbol "max")))
          `(let ((,var ,init)
                 (,tempvar ,final))
             (while (<= ,var ,tempvar)
               ,@body
               (inc ,var)))))
 
 This creates an uninterned symbol named ‘max’ and puts it in the
 expansion instead of the usual interned symbol ‘max’ that appears in
 expressions ordinarily.
 

Info Catalog elisp: Argument Evaluation elisp: Problems with Macros elisp: Eval During Expansion