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Request for comments: Printf list conversion
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Date: -- (:)
From: Hendrik Tews <tews@t...>
Subject: Re: [Caml-list] Request for comments: Printf list conversion
Bill Wood <william.wood3@comcast.net> writes:

   I'm always inspired in my hacks by Common Lisp's (format...) form, which
   includes a ~{/~} pair containing conversion directives and that consumes
   a list argument.  I don't want to appall you with complexity too soon,
   but when you think about this, consider the possibility that the list is
   a list of lists, each of which contains several items that are to be
   formatted using a a recursively specified (sub) format.  This does
   enhance usability a great deal.  I'm sure that there are people who will
   yelp in protest at this, but I do recommend looking at CL's format as at
   least one point in this design space.
   
I doubt anybody will yelp. Having a printf format for 'a list it
is just natural to permit arbitrary formats for the 'a elements
of the list. Therefore I would suggest something like

Printf.bprintf buf "int pair list: %[(%d, %d) %]" [ 1;2;3;4;5 ]

Here %[ is the list format and everything inbetween %[ and %] is
the format for the list elements (like the  %{ .. %} format). 

It remains to deal with separators that you don't want to print
after the last element. Maybe this could be solved by permitting
flags for %], like 

- flag space "% ]": use space separator, omitted after the last
                    element
- flag comma "%,]": similar for comma
- flag s     "%s]": use next argument as format and interpred
                    this format as separator

A flag that permits a format would also Virgile Prevosto problem:

Virgile Prevosto <virgile.prevosto@m4x.org> writes:

   Le jeu 12 jan 2006 13:55:56 CET, Damien Doligez a écrit:
   > 
   > Yuck.  Isn't this type (formatter -> unit -> unit) isomorphic to string,
   > in the absence of side-effects?  Or do you really need separators that
   > behave differently depending on the state of some reference?
   > 
   
   With Format, yes: you may want to use the pretty-printing features of the
   module, and for instance add a break hint after each element of the list.

Just use 

Format.printf "int pair list: %[(%d, %d)%s]" [ 1;2;3;4;5 ] "@;<2 2>"


BTW, what is the common list solution for printing separators
just between the list elements?

Bye,

Hendrik