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Question re: camlp4 parser
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Date: -- (:)
From: Paul Snively <psnively@m...>
Subject: Re: [Caml-list] Question re: camlp4 parser
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Hello, Stephane!

On Jul 25, 2005, at 6:17 PM, Stephane Glondu wrote:
>
> The inferred type should have given you a warning:
> --> val printable : char Stream.t -> 'a = <fun>
>
> In other word, your function never returns a correct value.
>
Excellent point.

> Try this:
>
> let printable s =
>   let buf = Buffer.create 100 in
>   let rec aux = parser
>       [< '' '..'~' as c; x = (Buffer.add_char buf c; aux) >] -> x
>     | [< >] -> Buffer.contents buf
>   in aux s ;;
> --> val printable : char Stream.t -> string = <fun>
>
> printable (Stream.of_string "Test!\013") ;;
> --> - : string = "Test!"
>
> Notice that you cannot remove the occurrences of "s" (even though it
> would have the same type) if you are planning to use this function
> several times.
>
Thanks, this is exactly the kind of thing I was hoping for! So the  
key points are:

1) Use the | and an empty alternative pattern to capture the "no more  
matches" case.
2) Use "as" and take advantage of expression sequencing to accumulate  
the matches into a variable (Buffer, in this case).

That makes perfect sense and now seems obvious. :-)

One hopefully final question: is there a convenient shorthand for  
saying something like "all printable characters except '=' or '['?" I  
assume not--that is, we have ranges (' '..'~') or we have variants  
('A' | 'B' | 'C'...) and that's it. I'm somewhat spoiled, I think, by  
Spirit in C++, and its notion of "character sets" and operations on  
them, so I can say, e.g. "print_p - '='" that that will match all  
printable characters other than '='.

>
>> Many thanks and best regards,
>>
>
> You're welcome.
>

Thanks again,

> -- 
>
> Stephane Glondu.
>
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>

Paul Snively

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