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Request for comments: Printf list conversion
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Date: -- (:)
From: Alessandro Baretta <a.baretta@b...>
Subject: Request for comments: Printf list conversion
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