[pro] User defined format functions

[Didier Verna]

  Hello,

there are situations in which I find myself frequently using the same
format "pattern", which I'd like to abstract away. A concrete example is
from my Declt package[1] which prints a lot in Texinfo format.

Since Texinfo has a couple of special characters (e.g. @), I have an
ESCAPE function which takes a string and returns another one with all
special characters escaped. In the Declt code, I hence have very
frequent calls which look like this:

(format stream "... ~A ..." #|...|# (escape str) #|...|#)


This is frustrating because I would like to abstract away the relation
"~A <-> #'escape". Of course there are solutions (I could provide my own
wrapper around FORMAT in some way) but none of them seem satisfactory.

I'm aware of the ~// construct, which I find extremely cumbersome. Does
anybody actually use it? It seems to me that the package handling part,
specifically, makes it totally unusable. Just imagine that I would need
to do something like that if I were to use it:

(format stream "... ~/com.dvlsoft.declt:escape/ ..." #|...|# str #|...|#)

and you get the idea! :-)


No, in fact, the problem is that FORMAT control sequences are not
extensible. This reminds me of something that we have in the Gnus
mail/newsreader. It's called "user format functions". Gnus Group and
Summary buffer lines, for example, are define by a format string with a
set of predefined %<char> constructs. One of them, %u<char> lets you
define and use your own function, called
gnus-user-format-function-<char>.


So what I would like to do is more or less define a function
FORMAT-FUNCTION-E, just like a standard formatter, and then be able to
write something like that:

(format stream "... ~\e\ ..." #|...|# str #|...|#)


Of course, there are package issues. Maybe we would need a centralized
mapping between function names (what goes in ~\\) and actual symbols
denoting the actual function? Possibly with the possibility to override
the mapping by providing a package prefix in the ~\\ construct?

In fact, the more I think about it, the more it sounds like having a
format-table facility, much like what a readtable is, with a similar
API. You could do (in-format-table :my-table-name) at the beginning of a
file and have a totally customized set of format control sequences,
possibly inheriting the standard ones.



WDYT? Does something like that already exists[2]?



Footnotes: 
[1]  http://www.lrde.epita.fr/~didier/software/lisp/misc.php#declt

[2]  I've heard of something called fmt for scheme, but don't know much
about it.

-- 
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