[Caml-list] is there a way for turning strings to "format"s?

[Caml-list] is there a way for turning strings to "format"s?

[Matej Kosik]

Hi,

I would like to ask, how to do something like:

	let computed_format = Printf.sprintf "%%0%dd" 5 in
	Printf.printf computed_format 42

The above code is rejected, because "computed_format" is of type "string" whereas

        ('a -> 'b, out_channel, unit) format =
           ('a -> 'b, out_channel, unit, unit, unit, unit) format6

type was expected.

My question is: is it possible to turn strings to values that would be accepted by Printf.printf as its first parameter?

Re: [Caml-list] is there a way for turning strings to "format"s?

[Simon Cruanes]

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Le Wed, 06 Nov 2013, Matej Kosik a écrit :

> Hi,
> 
> I would like to ask, how to do something like:
> 
> 	let computed_format = Printf.sprintf "%%0%dd" 5 in
> 	Printf.printf computed_format 42
> 
> The above code is rejected, because "computed_format" is of type "string" whereas
> 
>         ('a -> 'b, out_channel, unit) format =
>            ('a -> 'b, out_channel, unit, unit, unit, unit) format6
> 
> type was expected.
> 
> My question is: is it possible to turn strings to values that would be accepted by Printf.printf as its first parameter?

I don't think you can in general, because format strings are typed (so
that the compiler can check the correctness of Printf.printf calls).

However, you could try something along

let computed_format = Printf.sprintf "%%0%d%d" 5 in
print_string (computed_format 42)

-- 
Simon

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Re: [Caml-list] is there a way for turning strings to "format"s?

[Robert Jakob]

> My question is: is it possible to turn strings to values that would
> be accepted by Printf.printf as its first parameter?

You might be able to use Scanf.format_from_string:
http://caml.inria.fr/pub/docs/manual-ocaml/libref/Scanf.html#VALformat_from_string

r.

Re: [Caml-list] is there a way for turning strings to "format"s?

[Jeremy Yallop]

On 6 November 2013 12:39, Matej Kosik
<5764c029b688c1c0d24a2e97cd764f@gmail.com> wrote:
> I would like to ask, how to do something like:
>
>         let computed_format = Printf.sprintf "%%0%dd" 5 in
>         Printf.printf computed_format 42

For this example, you can use '*' in the format string to pass the
width as an argument

         let computed_format = Printf.sprintf "%0*d" 5 in
         computed_format 42

or even just

         Printf.sprintf "%0*d" 5 42

Re: [Caml-list] is there a way for turning strings to "format"s?

[Matej Kosik]

Guys,

Thanks for all the answers.

On 06/11/13 12:57, Jeremy Yallop wrote:
> On 6 November 2013 12:39, Matej Kosik
> <5764c029b688c1c0d24a2e97cd764f@gmail.com> wrote:
>> I would like to ask, how to do something like:
>>
>>         let computed_format = Printf.sprintf "%%0%dd" 5 in
>>         Printf.printf computed_format 42
> 
> For this example, you can use '*' in the format string to pass the
> width as an argument
> 
>          let computed_format = Printf.sprintf "%0*d" 5 in
>          computed_format 42
> 
> or even just
> 
>          Printf.sprintf "%0*d" 5 42
> 

This is great! \o/

Re: [Caml-list] is there a way for turning strings to "format"s?

["Mark Adams"]

There's always the nasty 'Obj.magic' for conversion between arbitrary types:

    let computed_format = Printf.sprintf "%%0%dd" 5 in
    Printf.printf (Obj.magic computed_format) 42

Surely there must be a nicer way of doing this, but I don't know of one.

Mark.

on 6/11/13 12:40 PM, Matej Kosik <5764c029b688c1c0d24a2e97cd764f@gmail.com>
wrote:

> Hi,
>
> I would like to ask, how to do something like:
>
> let computed_format = Printf.sprintf "%%0%dd" 5 in
> Printf.printf computed_format 42
>
> The above code is rejected, because "computed_format" is of type "string"
> whereas
>
> ('a -> 'b, out_channel, unit) format =
> ('a -> 'b, out_channel, unit, unit, unit, unit) format6
>
> type was expected.
>
> My question is: is it possible to turn strings to values that would be
> accepted by Printf.printf as its first parameter?
>
> --
> Caml-list mailing list.  Subscription management and archives:
> https://sympa.inria.fr/sympa/arc/caml-list
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> Bug reports: http://caml.inria.fr/bin/caml-bugs
>
>
>

Re: [Caml-list] is there a way for turning strings to "format"s?

[Gabriel Scherer]

> There's always the nasty 'Obj.magic' for conversion between arbitrary types:
>
>     let computed_format = Printf.sprintf "%%0%dd" 5 in
>     Printf.printf (Obj.magic computed_format) 42

This is terribly wrong. Not only does it remove any form of static
typing on today's version of OCaml, it is also assured to segfault as
soon as the format representation changes (which may happen in a close
future). Do not use Obj.magic.

Matej: Deciding the number of decimal points or 0-padding dynamically
is one of those eccentric features that are in fact already part of
format.

  # Printf.printf "%0*d" 6 2;;
  000002- : unit = ()

Format will be your friend as long as you stay inside the things they
directly support -- look at the documentation carefully to discover
some surprisingly expressive features. If you ever step outside this
zone, the temptation is great to hack your way back in, but that will
only lead to trouble (consider simply using OCaml code to implement
your printing logic then). Do not, ever, use Obj.magic on formatting
functions.

On Wed, Nov 6, 2013 at 1:54 PM, "Mark Adams"
<mark@proof-technologies.com> wrote:
> There's always the nasty 'Obj.magic' for conversion between arbitrary types:
>
>     let computed_format = Printf.sprintf "%%0%dd" 5 in
>     Printf.printf (Obj.magic computed_format) 42
>
> Surely there must be a nicer way of doing this, but I don't know of one.
>
> Mark.
>
> on 6/11/13 12:40 PM, Matej Kosik <5764c029b688c1c0d24a2e97cd764f@gmail.com>
> wrote:
>
>> Hi,
>>
>> I would like to ask, how to do something like:
>>
>> let computed_format = Printf.sprintf "%%0%dd" 5 in
>> Printf.printf computed_format 42
>>
>> The above code is rejected, because "computed_format" is of type "string"
>> whereas
>>
>> ('a -> 'b, out_channel, unit) format =
>> ('a -> 'b, out_channel, unit, unit, unit, unit) format6
>>
>> type was expected.
>>
>> My question is: is it possible to turn strings to values that would be
>> accepted by Printf.printf as its first parameter?
>>
>> --
>> Caml-list mailing list.  Subscription management and archives:
>> https://sympa.inria.fr/sympa/arc/caml-list
>> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
>> Bug reports: http://caml.inria.fr/bin/caml-bugs
>>
>>
>>
>
> --
> Caml-list mailing list.  Subscription management and archives:
> https://sympa.inria.fr/sympa/arc/caml-list
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> Bug reports: http://caml.inria.fr/bin/caml-bugs

Re: [Caml-list] is there a way for turning strings to "format"s?

[David MENTRE]

Hello,

2013/11/7 Gabriel Scherer <gabriel.scherer@gmail.com>:
> Do not, ever, use Obj.magic on formatting
> functions.

I would say: "Dot not, ever, use Obj.magic (FULL STOP)". ;-)

BTW, isn't "ksprintf " and related functions used when one wants to
play with format strings?

Best regards,
david

Re: [Caml-list] is there a way for turning strings to "format"s?

[Ollie Frolovs]

When i first saw this question i had a deja vu moment and as was flicking through “Real World Ocaml” today i found this example in pages 173-174: 

#let fmt = : (‘a, ‘b, ‘c) format = “%i is an integer, %F is a float, \”%s\” is a string\n”;;
val fmt = (int -> float -> string -> ‘c, ‘b, ‘c) format = <abstr>

and then they say you can pass this format string to printf. I don’t know if they mean Jane Street’s Core printf or stock printf. I’m only a beginner myself.

— Ollie

On 7 Nov 2013, at 07:50, David MENTRE <dmentre@linux-france.org> wrote:

> Hello,
> 
> 2013/11/7 Gabriel Scherer <gabriel.scherer@gmail.com>:
>> Do not, ever, use Obj.magic on formatting
>> functions.
> 
> I would say: "Dot not, ever, use Obj.magic (FULL STOP)". ;-)
> 
> BTW, isn't "ksprintf " and related functions used when one wants to
> play with format strings?
> 
> Best regards,
> david
> 
> -- 
> Caml-list mailing list.  Subscription management and archives:
> https://sympa.inria.fr/sympa/arc/caml-list
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> Bug reports: http://caml.inria.fr/bin/caml-bugs

Re: [Caml-list] is there a way for turning strings to "format"s?

[Yaron Minsky]

It should work for both.

On Thu, Nov 7, 2013 at 3:45 PM, Ollie Frolovs <of12343@my.bristol.ac.uk> wrote:
> When i first saw this question i had a deja vu moment and as was flicking through “Real World Ocaml” today i found this example in pages 173-174:
>
> #let fmt = : (‘a, ‘b, ‘c) format = “%i is an integer, %F is a float, \”%s\” is a string\n”;;
> val fmt = (int -> float -> string -> ‘c, ‘b, ‘c) format = <abstr>
>
> and then they say you can pass this format string to printf. I don’t know if they mean Jane Street’s Core printf or stock printf. I’m only a beginner myself.
>
> — Ollie
>
> On 7 Nov 2013, at 07:50, David MENTRE <dmentre@linux-france.org> wrote:
>
>> Hello,
>>
>> 2013/11/7 Gabriel Scherer <gabriel.scherer@gmail.com>:
>>> Do not, ever, use Obj.magic on formatting
>>> functions.
>>
>> I would say: "Dot not, ever, use Obj.magic (FULL STOP)". ;-)
>>
>> BTW, isn't "ksprintf " and related functions used when one wants to
>> play with format strings?
>>
>> Best regards,
>> david
>>
>> --
>> Caml-list mailing list.  Subscription management and archives:
>> https://sympa.inria.fr/sympa/arc/caml-list
>> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
>> Bug reports: http://caml.inria.fr/bin/caml-bugs
>
>
> --
> Caml-list mailing list.  Subscription management and archives:
> https://sympa.inria.fr/sympa/arc/caml-list
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> Bug reports: http://caml.inria.fr/bin/caml-bugs