[m-users.] DCGs: equivalent of Prolog's phrase/2 and phrase/3 predicates?

[Sean Charles (emacstheviking)]

Before setting off to the tattoo parlour I made one small change to keep it more ‘DCG’, I removed {true} and put in [] instead./..

skip_ws -->
    ( if   ([' '];['\t'])
      then skip_ws
      else []
    ).

Nice work though!

> On 8 May 2021, at 16:54, Volker Wysk <post at volker-wysk.de> wrote:
> 
> Am Samstag, den 08.05.2021, 16:26 +0100 schrieb Sean Charles
> (emacstheviking):
>> 
>> In the end I made it `set` like this, I don’t see why skipping whitespace should fail anyway, if it isn’t there then the next part of the calling rule can deal with it!
>> 
>> :- pred skip_ws(list(char)::in, list(char)::out) is det.
>> 
>> skip_ws(In, Out) :-
>>    trace[io(!S), run_time(env("FELT_DBG"))]
>>    (io.format("skip_ws: ",[],!S),
>>     io.print_line(In,!S)
>>    ),
>>    ( if [C|Cs] = In then
>>        ( list.member(C, [' ', '\t']) ->
>>            skip_ws(Cs, Out)
>>        ;
>>            Out =In
>>        )
>>        else
>>            Out = In
>>    ).
> 
> This could be written like this:
> 
> :- pred skip_ws(list(char)::in, list(char)::out) is det.
> skip_ws -->
>    ( if ([' '] ; ['\t'])
>      then skip_ws
>      else { true }
>    ).
> 
> 
> Cheers,Volker

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