Re: [Caml-list] Request for comments: Printf list conversion

[Jean-Marc EBER <jeanmarc.eber@lexifi.com>]

Just for info:

Not identical of course, but related is

  0003882: Wish: Format.pp_print_list

in the bug tracker (and wish list) system of ocaml.

Jean-Marc

Alessandro Baretta wrote:

> I would like to transform the following rather vague idea into a formal 
> feature request. Before doing this, I would like to know what others 
> think about it.
> 
> I continually come across the need to pretty-print via Printf a list of 
> values--for example, let's say an int list. Printf has conversion 
> specifiers for int, but int list cannot be handled easily. What I 
> usually end up doing is converting values in the list to strings, 
> concatenate them, and output the result with the %s conversion specifier.
> 
> Printf.bprintf buf "A beatiful list of ints: [ %s ]\n" (String.concat " 
> ;" (List.map string_of_int [ 1;2;3;4;5 ] )
> 
> Obviously, this is radically ineffiecient. A better solution might be 
> the following.
> 
> Printf.bprintf buf "A beatiful list of ints: [ %t ]\n" (fun buf ->
>   List.iter (Printf.bprintf buf " %d;") [ 1;2;3;4;5 ]
>   )
> 
> The above is adequately fast, but terribly ugly.
> 
> I believe that it should be possible to add a conversion-flag--let me 
> use '&' as an example--telling the type-system and the Printf module 
> that the corresponding parameter is a list of values whose type is 
> determined by the conversion-specifier proper. Let me show what I would 
> like to be able to write:
> 
> Printf.bprintf buf "A beatiful list of ints: [ %&d ]\n" [ 1;2;3;4;5 ]
> 
> Here, the &-flag would tell the type-system that the first parameter is 
> an 'a list. The fact that 'a = int is to be inferred from the actual 
> conversion specifier ('d'). Upon recognizing a %& conversion, Printf 
> would have to iterate over the corresponding parameter with the 
> appropriate conversion (format_int_with_conv, in this case).
> 
> Now, of course, the '&' conversion-flag does not settle the issue, as a 
> parameter like the first one of String.concat would also be needed. 
> Another issue which is brought up by the idea I have jotted down is how 
> to handle conversions like '%&a' and '%&t'.
> 
> What do you guys think about all this?
> 
> Alex
> 
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