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

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

> 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?
>>
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>
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