[wg-camlp4] [Caml-list] Working Group: the future of syntax extensions in OCaml, after camlp4

[Hongbo Zhang]

IIRC, the stream parser is in the old caml, but it is removed later since
Daniel write it in camlp4 with domain specific optimizations

On Mon, Jan 28, 2013 at 4:12 PM, Gerd Stolpmann <info at gerd-stolpmann.de>wrote:

> Am 28.01.2013 13:15:06 schrieb(en) Alain Frisch:
>
>  On 01/24/2013 05:24 PM, Gerd Stolpmann wrote:
>>
>>> It's used in the tool, but only for stream parsing. I could also
>>> distribute the already-preprocessed file (and maybe I'll do so in the
>>> next release).
>>>
>>> Stream parsing is certainly one of the topics to discuss.
>>>
>>
>> I've no idea how widely stream parsing is used.  Has anyone some
>> intuition about this?
>>
>> Stream parsers probably fall in the same category as bitstring or sedlex
>> (custom notions of pattern matching).  It seems that stream parsers (which
>> I'm not familiar with) require to be able to write expressions within
>> "left-hand sides", which might require special support.  Or maybe the whole
>> left-hand sides should just be quotations.
>>
>> Anyway, for a basic infrastructure tool such as ocamlfind, I'd probably
>> advocate for a "manual" solution which works out of the box with a basic
>> OCaml installation (ocamlyacc or manual top-down parser).  Gerd: does that
>> sound reasonable to you?
>>
>
> Of course. META is easy enough to parse with any tool.
>
> But I'd also like to fight for stream parsers a bit. In general, they are
> a nice way to write top-down parsers. Like for all top-down parsers, the
> class of acceptable grammars is not very powerful (less than LALR(1)), but
> you can easily add things that are nearly impossible with bottom-up
> parsers, like stores or contextuality, and you can locally escape from the
> parsing discipline entirely. Sometimes it is a perfect match.
>
> From an old OCaml manual:
>
> expr ::= ...
>         | [< >]
>         | [< stream-component {; stream-component} >]
>         | parser [pattern] stream-matching
>         | match expr with parser [pattern] stream-matching
>
> stream-component ::= ’ expr
>                    | expr
> stream-matching ::= stream-pattern [pattern] -> expr
>                     { | stream-pattern [pattern] -> expr}
> stream-pattern ::= [< >]
>                   | [< stream-pat-comp {; stream-pat-comp [?? expr]} >]
> stream-pat-comp ::= ’ pattern [when expr]
>                   | pattern = expr
>                   | ident
>
> Doing this with -ppx is certainly a challenge.
>
> Some ideas:
>
>  - [< >]: Maybe we should simply add several types of brackets to
>    the grammar. Syntax as for list, but reserved for extensions.
>
>  - 'expr: This is like a tag. Maybe use I(expr) instead, where "I"
>    means "item".
>
>  - pattern = expr: I guess this is what Alain sees as difficulty.
>    As quotation it would look strange. What we need here is something
>    in the Parsetree that allows us to include expressions into patterns
>    (which would look like a "computed pattern"). Maybe `(expr) inside
>    patterns. Or another notation...
>
> Gerd
>
>
>
>>
>> Alain
>>
>>
>>
>
> --
> ------------------------------**------------------------------
> Gerd Stolpmann, Darmstadt, Germany    gerd at gerd-stolpmann.de
> Creator of GODI and camlcity.org.
> Contact details:        http://www.camlcity.org/**contact.html<http://www.camlcity.org/contact.html>
> Company homepage:       http://www.gerd-stolpmann.de
> ------------------------------**------------------------------
>
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-- 
-- Regards, Hongbo
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