Re: [Caml-list] [batteries] ... how to create (format) directives that do not take any arguments?

[Matej Kosik <5764c029b688c1c0d24a2e97cd764f@gmail.com>]

On 25/11/13 16:27, Gabriel Scherer wrote:
> Adding custom printing directives requires a syntax extension to
> rewrite those directives into OCaml code. The built-in support for
> magicly-typed formats in the OCaml language is not extensible (it is
> complex enough as it is).
> Batteries 1 used to provide such a syntax extension, but we
> (re)discovered that users don't like syntax extensions: they make code
> harder to compile/deploy, are controversial, and in the long run often
> make our life harder instead of easier. Batteries 2 does not come with
> any syntax extension, and I think we're better off as is (of course
> you're free to add your own in your code).
> 
> Of course, such a natural idea is bound to be reused, and one of the
> first libraries released by Jérémie Dimino at Jane Street was
> precisely such a custom-printf camlp4 extension:
>   https://github.com/janestreet/custom_printf
> 
> Feel free to reuse it or extend it for any project where it makes
> sense. In particular, I don't think that using it implies any
> dependency of your preprocessed code on Core, though of course you'll
> need Sexplib to use the format-to-sexplib feature. My purely personal
> advice would be to seriously explore non-syntax-extension approaches
> (combinator library?) before making such a step.

I may, unfortunatelly, need some more pointers to understand this suggestion.

The problematic code looks somehow like this:


        (* The following string is much longer than what is inlined here. *)

        bold_on ^ "mdx" ^ bold_off ^ " " ^ underline_on ^ "command" ^ underline_off ^ "
        Perform a given " ^ underline_on ^ "command" ^ underline_off ^ ".
        Section COMMANDS describes all the supported commands."

where:

	bold_on
	bold_off
	underline_on
	underline_off

are some strings.

It works, but it is ugly.

That is why I asked about possibilities to make it more decent.

Since Print module is available, I thought that that may be one of the ways how to achieve that.
If only I were able to define (non-parametric control strings) that I could use in the following way:

	"%Bmdx%B %Icommand%I

	 Perform a given %Icommand%I.

         Section COMMANDS describes all the supported commands."

This looks, sort of, OK.

I believe that the Print module in batteries 1.4 might enable me to do just that although I need some examples.
I do not care too much that given feature disappeared in Batteries 2.0
(Batteries 2.0 is not yet available in Debian stable/testing/unstable
 and when in descends there, that time might be beyond the lifecycle of the program I am writing.
 If not, I will bother about it at right time. There might even be more solutions than there are now.).

If you mean that it is not technically possible to do what I thought was possible,
then I am not against "combinator library", although I am not sure, what do you mean.

I would be grateful if you (or somebody) sent a few links to things that might be regarded as combinator libraries
and perhaps, if given combinator library existed, that would solve my problem,
how it would make my_life_easier & my_code_more_compact than the (somewhat verbose) approach I am currently using
(string concatenation).