Hi,

Could someone tell me why, say "%s", is of type

  'a. (string -> 'b, 'a, 'b) format

instead of

  'a 'b. (string -> 'b, 'a, 'b) format

I am asking this because if one wants to use the same format string
both for reading and printing in a given function, one needs the
latter type:

  type 'a fmt = { fmt: 'b 'c. ('a,'b,'c) format }
  fun  (s: _ fmt) -> Printf.printf s.fmt, Scanf.sscanf "string" s.fm

However, I cannot instantiate the type [_ fmt].  This is in contrast
with

  let s = ("%s": (_,_,_) format) in
  Printf.printf s, Scanf.sscanf "string" s

which works as intended.  I think this may have been discussed before;
I just can't remember the reason.  Also, is there a safe way of using
Obj.magic to make it work ?

Regards,
ChriS

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From: Christophe TROESTLER <Christophe.Troest...@umh.ac.be>

It clearly has the second type!

With your definition of fmt, you are requiring 'c to be independent
from 'a. But If you look at the type of "%s", you see that
'a = string -> 'c, so that {fmt="%s"} is not well-typed.

A solution is to define it as
  type 'a fmt = {fmt: 'b 'c. ('a -> 'b, 'c, 'b) format}

The drawback is that only allows formats taking one argument.
One could add definitions for any defined number of arguments, but I
don't see any generic solution.

Cheers,

Jacques Garrigue

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Thanks for answering -- I got badly confused by the error message !
(shame on me :)

Well, yes, of course, that's obvious after you stated it!  :)

Yeah but I really want to allow as many arguments as the user desires.
That's a bit of an annoyance for printf/scanf like schemes: you have
to know in advance in how many contexts you may want to simultaneously
use an argument of type format in order to put enough type parameters...

So I suppose my best luck to do it cleanly is to use camlp4 to make
the parameter duplication for the user.  Another approach I was
thinking about is to define a new type, say

  type ('a,'b,'c,'d) fmt

and have (for the above application)

  external pr_fmt : (_, _, 'a, 'c) fmt -> ('a, out_channel, 'c) format = "%identity"
  external sc_fmt : ('a, 'c, _, _) fmt -> ('a, Scanf.Scanning.scanbuf, 'c) format = "%identity"
  external ( !/ ) : ('a, _, 'b) format -> ('a, 'b, 'a, _) fmt = "%identity"

  let f s =
    Printf.printf (pr_fmt s), Scanf.sscanf "string" (sc_fmt s)

  f !/"%s %u"

Now, of course ( !/ ) is not completely doing what I want.  If
[pr_fmt] and [sc_fmt] are for purely internal use by the library --
thus user only seeing [/!] -- is the above safe? Is there a way using
Camlp4 to reinterpret a string in the new type (possibly prefixing it
with !/ or another chosen symbol to distinguish it) so that,
say, !/"%i %f" becomes (Obj.magic "%i ... %f": (int -> float -> 'a,
'a, int -> float -> 'b, 'b) fmt) ?

Thanks for your help,
ChriS

P.S. Out of curiosity, is there a type system with inference (say,
extending the one of OCaml) that can handle this kind of thing cleanly?

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