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Date: Mon, 29 Mar 1999 18:31:50 +0200
From: Xavier Leroy <Xavier.Leroy@inria.fr>
To: John Prevost <prevost@maya.com>, caml-list@inria.fr
Subject: Re: another approach to sprintf (Re: ocaml 2.02 bug: curried printf)
References: <19990312160017.60444@pauillac.inria.fr> <19990319174720-10695P.sumii@harp.is.s.u-tokyo.ac.jp> <19990323171753.17334@pauillac.inria.fr> <ya2pv5ywuem.fsf@zarya.maya.com>
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In-Reply-To: <ya2pv5ywuem.fsf@zarya.maya.com>; from John Prevost on Wed, Mar 24, 1999 at 02:37:53PM -0500
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> It does, however, mean that people can extend the set of patterns that
> can be used in printf in a more palatable way than the %a mechanism.
> Especially with neat things like Danvy's "lis" combinator.

This is a good point.

> It also allows me to take two formats and concatenate them, like this:
> let foo = int $ lit " " $ int $ string
> let bar = lis int $ lit "!"
> let zum = foo $ bar
> which you can't do with O'Caml's format strings.

You almost can.  The following definition works as long as you
don't have %a and %t escapes in your format strings:

let (^^) (s1 : ('a, 'b, 'c) format) (s2 : ('c, 'b, 'd) format) =
  (Obj.magic (Obj.magic s1 ^ Obj.magic s2) : ('a, 'b, 'd) format)

If you have occurrences of %a or %t in s1, the typing becomes wrong.
This could be fixed by adding a fourth type parameter to the "format"
type constructor, but I agree this is getting really complicated.

- Xavier Leroy