Re: [m-users.] parsing, seeking advice on simple combinator project and also operators.

Hi Ju8lien,

Yes, I was already aware of that,. part of my question was, "What modules define those operators?" as it would make it easier to try to re-use one.

If I had included 'int', and still wanted to use, '<' for my own purposes, I am assuming that I could use 

:- use_module int.

and then qualify the real '<' somehow, not sure what the synta would be

    X int.'<' Y    perhaps?

Anyway, I am just going to keep reading, keep trying small test examples until it sinks into my thick skull.

Thanks all,
Sean.



> On 30 Oct 2022, at 02:33, Julien Fischer <[email protected]> wrote:
> 
> 
> 
> On Sat, 29 Oct 2022, Sean Charles (emacstheviking) wrote:
> 
>> I have spent a goodly few days on and off reading the xml.parser.m file in the extras folder for some insights but whoever wrote that is light years ahead of
>> me, it's been hard to follow at best.
>> I also returned to reading the code for mmc-doc, another good source of inspiration, in particular I tried to adapt this code but I don't think I filly
>> understand yet how Mercury does higher-order functions, nor do I fully understand modes/insts when related to HOP.
>> Here is the code, it provides (file: config.m) a function '//' that allows chaining, which is what I am trying to do:
>> :- type maybeio == (pred(maybe(string), io, io)).
>> :- inst maybeio == (pred(out, di, uo) is det).
>> :- func (maybeio::in(maybeio)) // (maybeio::in(maybeio))
>>     = (maybeio::out(maybeio)).
>> A // B = C :-
>>     C = (pred(Res::out, !.IO::di, !:IO::uo) is det :-
>>         A(Res1, !IO),
>>         (
>>             Res1 = Res @ yes(_)
>>         ;
>>             Res1 = no,
>>             B(Res, !IO)
>>         )).
>> I can see that the chain is started further down:
>>         GetConfig = get_environment_var("XDG_CONFIG_HOME")
>>             // get_environment_var("APPDATA")
>> :
>> :
>> I see that the definition of '//' is such that it expects predicates on both sides and that they are executed -inside- the implementation of the function i.e.
>> the get_environment_var() is not called at that point in the code but is in fact passed in as an argument to '//', and then executed by the line 'A(Res1, !IO)'
>> or 'B(Res,!IO)', that much I have understood. Hope resigns supreme.
>> How did the author know to use '//'?  Given that Mercury doesn't allow arbitrary strings of non-alpha characters to be used as a function/operator name like
>> Prolog. On page 6, section 2 'Operators' (reference manual), there is a huge list of operators but it isn't obvious where to look for the module that defines
>> them, I am assuming that '//' is used in a module NOT imported into the source file but is recognised by the parser? How does one know what operators can be
>> used for ones own purposes, I managed to find that '//' and '**' worked for me but I am not sure what they do, I am guessing that '**' is 'power' or
>> 'exponential' ?
> 
> That section of the reference manual was re-written recently.  If you
> have a look at the ROTD version, you'll find the explanation:
> 
>    Note that operators are a syntactic concept. The ‘+’ infix operator, for
>    example, is only a symbol; it does not mean addition unless you write or
>    import code that defines it as addition. Modules in the Mercury standard
>    library, such as int and float, provide such arithmetic definitions. To
>    illustrate further, the ‘-’ infix operator is defined as subtraction by
>    those modules but is defined as a pair constructor by the pair module.
> 
> Julien.

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