I noticed that there is no
format directive which would
call force-output/finish-output.
Why?
It does seem to be useful in user interaction, cf. Lisp format and force-output.
E.g., ~= could translate to finish-output, and ~:= to force-output.
I don't think clear-output makes much sense in this context, but we
might map ~@= to it for completeness.
PS. Cf. CLISP RFE.
Summary from comp.lang.lisp:
The language defines no portable way to extend the set of format
directives (other then ~/.../) but that's not really the issue here.
The real problem is that it is not well defined to call finish-output or similar functions at arbitrary places during printing.
If pretty-printing is in progress, the stream received by a
pprint-dispatch or print-object method may be an encapsulating stream --
one that delays output temporarily until it can make decisions about
white space and line breaks. (There are also potential problems if
finish-output were called inside a ~< justification, but that directive
is a hairball!) What would one expect finish-output to do if called
inside a pretty print operation? I don't think it's well defined.
The problem isn't particular to format, of course, but a directive for
finish-output from format would just add another sharp edge to the
language. finish-output etc. are only safe to call when completely
outside an actual or implied call to cl:write. Call it as a function
at an appropriate point in your code (where you know execution isn't
inside a nested write) so the intention is clear and you don't mess up
printer internals.
Actually, no changes to format are needed. Just add this function somewhere in the COMMON-LISP-USER package:
(defun fo (stream arg colon-p atsign-p &rest params)
(declare (ignore arg params))
(cond
(colon-p (force-output stream))
(atsign-p (clear-output stream))
(t (finish-output stream))))
Then:
(progn
(format t "enter var: ~/fo/" nil)
(read))
enter var: 456
456
The problems with this (portable!) approach are
~/fo/ instead of ~=)nil in the example above)