[m-dev.] io syntax change

[Ralph Becket]

> Dominique and I were discussing today about using the ideas from the
> purity syntax for predicates which do io.
> 
> For example:
> 
> :- io pred write_char_list(list(char)::in) is det.
> :- io pred write_char(list(char)::in) is det.
> 
> write_char_list([]).
> write_char_list([H|T]) :-
>     io write_char(H),
>     io write_char_list(T).
> 
> The advantages of this syntax are:
>     * it is quite a simple syntactic transformation
>     * it frees up the DCG notation for some other state
>     * in a lot of ways io is such a special case that IMHO having a
>       seperate syntactic construct is a good thing.
>     * I find it quite light weight
> 
> The disadvantages are
>     * I haven't thought about it a lot
>     * does it interact well with higher order code

This sounds like a special case of the extended DCG (EDCG) stuff somebody
was talking about way back when.

I can't remember exactly what the syntax was, but the annotation
went entirely on the pred declarations, something like:

:- edcg_args io == (io__state, io__state).
:- edcg_mode io == (di, uo).

:- pred io__write_string(string) + io.
:- mode io__write_string(in) + io is det.
...

:- pred foo(string) + io.
:- mode foo(in) + io is det.

foo(S) -->>
	io__write_string("here's a string: "),
	io__write_string(S),
	io__nl.

A pred could have multiple hidden arguments.

My main objection to the scheme was that you had to know which
hidden arguments were changed by each pred; I like your idea of
explicitly annotating calls, so we might have something like
this:

:- pred bar(string) + acc + io.
:- mode bar(in) + acc + io is det.

bar(S) -->>
	[io] io__write(""blah"),
	[acc] do_something(S),
	[io, acc] finish_up.

Another problem (IMHO) was the syntax for accessing hidden args
directly was pretty ugly (definite Perl influences).

Ralph
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