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.