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eval'd macros (the other point of view)
Date: Mon, 3 Feb 1986 20:36 EST
From: "Scott E. Fahlman" <Fahlman@C.CS.CMU.EDU>
(defun fn () (mac)) ;==> FN
(defmacro mac () `'foo) ;==> MAC
(fn) ;==> ***Error***
I wish we'd required that this signal an error. I like the behavior shown above.
You don't say whether the problem you described occurs in the
interpreter or the compiler. Your example certainly is not required to
work when compiled because the macro definition follows the use. In my
opinion, this should work without an error in the interpreter. Hacking
in the interpreter would be fairly awkward if it didn't. However, the
fourth paragraph on page 143 seems to give implementors permission to do
compiler-like macro expansion when a defun is first seen. I wouldn't
want to use such an implementation, but it probably is legal as the
manual currently stands.
Well, people frequently don't like to use something that behaves differently
than what they're used to, but that doesn't make the `new' behavior wrong.
In the interest of fairness, let me outline the alternate point of view -- the
view to which I subscribe...
Having macros expanded when the defun is first seen offers the nice feature
that you can't easily break functions you've already written as you enter a
debugging phase. For example, consider how redefining LET would break the world
in an environment where LET wasn't resolved at definition time. Compare that
to a world where you had to do things in order. You're always trading one thing
Also, in your preferred interpreter, allowing displacing macros to expand
lazily will mean that code which is fully loaded but only partially exercised
may behave inconsistently after redefinition of a macro, since only the
non-displaced calls will see the update. In the case of an early-binding
interpreter, the effect of the redefinition is trivially predictable.
Personally, I would prefer not to work in an interpreter that did lazy
expansion. The only weakness I see in the spec is that it doesn't guarantee