Re: [Caml-list] Generic printer patch

[Gabriel Scherer <gabriel.scherer@gmail.com>]

Martin, in the meantime, you can use Extlib's (Std.dump : 'a ->
string) function, which is also integrated into Batteries. `dump` does
not require any modification to the compiler or toolchain.

For those that are not familiar with it, 'dump' uses the low-level
representation of OCaml values to print a sensible string representing
a value. As it only uses information that is available at runtime, it
is much less precise that Jeremie's printer, as for example None,
false, [] and 0 will all be printed as 0 (they have the same runtime
representation).

Of course, the "right" way to print/marshal data without changing the
language is to build a printer for your type from printer combinators.
Such combinators are available for example:
- in Jane Street Core library ( a Sexpable interface in each datatype module )
- in Xavier Clerc's Kaputt testing framework :
http://kaputt.x9c.fr/distrib/api/Utils.html

You can also use metaprogramming (.. camlp4) to generate printer
functions from datatypes description automatically, using such
combinators. See:
- Markus Mottl's 'type-conv': http://hg.ocaml.info/release/type-conv
- Jeremy Yallop's 'deriving': http://code.google.com/p/deriving/

Of course, printing values magically is still easier: you don't have
to build the printer yourself, passing subtype printers when
necessary, etc. I think Std.dump is reasonable for quick hacks or
debugging usage.

On Thu, Dec 8, 2011 at 7:20 PM, Martin Jambon
<martin.jambon@ens-lyon.org> wrote:
> On 12/08/2011 04:00 AM, Romain Bardou wrote:
>>>> 2) Could you imagine to generalize it to Format.formatter or to
>>>> out_channel (without creating a string and concatenating)? Romain Bardou
>>>> add in the mantis tracker (I can't give you the bugtracking number since
>>>> mantis "is currently offline for maintenance") a feature wish for a new
>>>> conversion specification that can print anything. Do you think you can
>>>> fulfill is dream?
>>
>> Here is the feature request I proposed:
>>
>> http://caml.inria.fr/mantis/view.php?id=4956
>>
>> Here is the response by Pierre Weis:
>>
>> "This is a major feature wish that requires careful thinking and a lot
>> of work!
>>
>> Furthermore, we would not have a completely satisfactory solution in the
>> end (due to this <poly> catch all case that tend to propagate, as far as
>> you use polymorphic functions). The correct solution to get this feature
>> in its full glory is a major modification of the type system along the
>> lines of G'Caml.
>
> The feature we want is exactly Jeremie's "hack" (his words). We need
> this feature for debugging and displaying data in log files. This kind
> of data is almost never polymorphic, so there is no practical issue
> here. Also we don't need a standardized output format. However we would
> often like to truncate the data to a reasonable size.
>
> I understand that this feature could be replaced in the future by a more
> complete solution, but we would be happy if it were provided as an
> "experimental extension" of OCaml.
>
>
> Martin
>
>> In short, a natural feature wish in a strongly typed polymorphic
>> language; we had it in mind for decades; unfortunately, we are not yet
>> ready to offer it, even in the rather limited extent you proposed."
>>
>> In other words: what you did is awesome but I'm not sure that it will be
>> added in the trunk :(
>>
>> Cheers,
>>
>
>
> --
> Caml-list mailing list.  Subscription management and archives:
> https://sympa-roc.inria.fr/wws/info/caml-list
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> Bug reports: http://caml.inria.fr/bin/caml-bugs
>