[m-dev.] io syntax change

[Peter Ross]

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

Let me know what you think.
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