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- To: firstname.lastname@example.org
- Subject: issue EVAL-WHEN-NON-TOP-LEVEL
- From: Jon L White <email@example.com>
- Date: Wed, 18 May 88 02:02:52 PDT
- Cc: firstname.lastname@example.org
- In-reply-to: Sandra J Loosemore's message of Mon, 9 May 88 09:55:33 MDT <8805091555.AA22855@cs.utah.edu>
The early part of the Proposal: section probably ought to outline our
intent in giving ordinary macro semantics to EVAL-WHEN -- it is so that
an piece of source code will either be "processed" as if the body of the
eval-when is there, or as if it were not there, depending on the dynamic
state of the processor:
Original Source | Situation Applies | Situation Doesn't Apply
(defun foo (x) | (defun foo (x) | (defun foo (x)
(step1 x) | (step1 x) | (step1 x)
(eval-when (...) | (step2 x) | nil
(step2 x)) | (step2 x)) | (step3 x))
(step3 x)) | |
On can infer this from a careful reading of the proposal, but the
overwhelming simplicity of making eval-when a small macro is lost "in the
woods". This top-level view of the proposal ought to be said very early
in the proposal, rather than starting out with a myriad of details about
how the various situations are treated.
I note also that you implicitly assume a dynamic variable *compiling-p*?
This will have to be spelled out in detail, much as in my note of
25 Mar 88 23:21:23 PST "Eval-When -- a radical view"; simply being
"in the compiler" isn't enough since EVAL may have to bind this variable
to nil (or do the equivalent -- Rob points out that we don't really
want to tie down the implementation to an actual dynamic variable, but
rather to the semantics give by the example code).
I very much like the idea of shadowing eval-when with a macrolet in the
situation where there might be nested eval-when's and the outter one has
at least a 'compile' situation. In fact, the trial code I sent out in
the msg "Eval-When -- a radical view" doesn't work right; the attempt
to identify this circumstance with a dynamic binding of a state variable
(to :eval-before-compile) is wrong, since it really is a lexical effect
that is needed, not a dynamic one.
Finally, I had a bit of trouble following all the logic in all the
cond clauses of the sample defmacro for EVAL-WHEN. Could I offer a
rewrite, with effusive commentary, that breaks it down into basically
two cases: the "hard" case (presented first) and the "easy" one.
(defmacro eval-when (situations &body body)
(cond ((and *compiling-p*
(member 'compile situations))
;; Situation Applies: The hard case. Compiling, and evaluation
;; is required at compile time too.
(eval `(progn ,@body))
(if (member 'load situations)
;; Allow normal processing by 'compile-form-to-file',
;; except ignore sub requests for compile-time evaluation.
`(macrolet ((eval-when (situations &body body)
(if (member 'load situations)
;; or skip processing by 'compile-form-to-file'
(member 'load situations)
(member 'eval situations))
;; Situation Applies: The easy case. Just return the body code
;; for normal processing by 'eval' or 'compile-form-to-file'.
;; Hmmm, 'situation' just doesn't apply!
-- JonL --