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elisp: Eval During Expansion

 
 13.5.4 Evaluating Macro Arguments in Expansion
 ----------------------------------------------
 
 Another problem can happen if the macro definition itself evaluates any
 of the macro argument expressions, such as by calling ‘eval’ (
 Eval).  If the argument is supposed to refer to the user’s variables,
 you may have trouble if the user happens to use a variable with the same
 name as one of the macro arguments.  Inside the macro body, the macro
 argument binding is the most local binding of this variable, so any
 references inside the form being evaluated do refer to it.  Here is an
 example:
 
      (defmacro foo (a)
        (list 'setq (eval a) t))
      (setq x 'b)
      (foo x) ↦ (setq b t)
           ⇒ t                  ; and ‘b’ has been set.
      ;; but
      (setq a 'c)
      (foo a) ↦ (setq a t)
           ⇒ t                  ; but this set ‘a’, not ‘c’.
 
 
    It makes a difference whether the user’s variable is named ‘a’ or
 ‘x’, because ‘a’ conflicts with the macro argument variable ‘a’.
 
    Another problem with calling ‘eval’ in a macro definition is that it
 probably won’t do what you intend in a compiled program.  The byte
 compiler runs macro definitions while compiling the program, when the
 program’s own computations (which you might have wished to access with
 ‘eval’) don’t occur and its local variable bindings don’t exist.
 
    To avoid these problems, *don’t evaluate an argument expression while
 computing the macro expansion*.  Instead, substitute the expression into
 the macro expansion, so that its value will be computed as part of
 executing the expansion.  This is how the other examples in this chapter
 work.
 

Info Catalog elisp: Surprising Local Vars elisp: Problems with Macros elisp: Repeated Expansion