[m-users.] Using type constructors in predicate heads

[Sean Charles (emacstheviking)]

I had actually done that earlier Zoltan by declaring a type

:- type callable —> callable(string, list(snode)).

but I felt that this was me giving in to the compiler. Thanks again for your feedback though, today I learned a new word,

 superhomogeneous

which feels like it is to do with milk but as we know it isn't.

Great list,
Thanks all once again,
Sean.


> On 7 Nov 2021, at 13:45, Zoltan Somogyi <zoltan.somogyi at runbox.com> wrote:
> 
> 
> 2021-11-07 15:11 GMT+11:00 "Julien Fischer" <jfischer at opturion.com>:
>>>> :- type snode
>>>     --->    sexp(location, list(snode))
>>>     ;       list(location, list(snode))
>>>     ;       map(location, list(snode))
>>>     ;       tk(location,string)
>>>     ;       kw(location, string)
>>>     ;       s1(location,string)
>>>     ;       s2(location,string)
> 
> Julien answered your question as you asked it, but there is
> something else you could do to achieve your goal that falls outside
> the strict parameters of your question: changing the snode type
> to create single structure holding all the info about sexp's.
> Specifically, you could define a new type like this:
> 
> :- type sexp_info
>    --->     sexp_info(location, list(snode)).
> 
> and then update the snode type like this:
> 
> :- type snode
>    ---->    sexp(sexp_info)
>    ;           ....
> 
> This way, after you have tested that a variable of type snode
> is bound to sexp, you could pass an sexp_info to other predicates;
> you wouldn't have to pass the whole snode.
> 
> I find this solution to be better when there is a lot of code
> that needs to work with the info associated with one function symbol
> (sexp in this case) that does not care about the other function symbols
> in that type. Of course, I don't know whether that is the case in your application.
> 
> Zoltan.

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