[m-dev.] EDCGs and Higher Order Programming

[Simon Taylor]

Fergus wrote:
> On 26-Jan-2000, Ralph Becket <rbeck at microsoft.com> wrote:
> > now we have the record
> > syntax, surely that (plus DCGs) should be sufficient to
> > achieve the same thing?
> 
> Good question.  But I don't think it works out quite so nicely.
> For example, suppose you want to pass the io__state and something
> else, so you put them both in a record.  Now you want to call
> something simple like `io__write_string' or `io__write_int'.
> To do that, you need to extract the io__state from the record,
> pass it to io__write_string and/or io__write_int,
> and then set the resulting io__state:


> > > 	write_strings(Strs) -->>
> > > 		WriteStringPred = (pred(S::in, A0::di, A::uo) is det :-
> > > 			(io is changed(A0, A) -->> io__write_string(S))),
> > > 		list__foldl(WriteStringPred, Strs, $io, $=io).

> Well, you can do a little better by abstracting out the conversion
> function:
> 
> 	:- func io2p((pred)+hidden(changed(io))) = pred(io__state, io__state).
> 	:- mode io2p(in(pred)) = out(pred(di, uo) is det).
> 	io2p(P) = (pred(IO0::di, IO::uo) is det :-
>  			(io is changed(IO0, IO) -->> P)).
> 
> Then you can write `write_strings' as
> 
> 	write_strings(Strs) -->>
> 		list__foldl(io2p(io__write_string), Strs, $io, $=io).

If the mode system were a bit more expressive, you could do a similar
thing for records containing `io__state's. With the current mode
system filling in the `XXX's is a bit difficult.

:- typeclass contains_io_state(T) where [
		func io_state(T) = io__state,
		mode io_state((XXX -> XXX_no_io)) = uo,

		func 'io_state:='(T, io__state) = T,
		mode 'io_state:='(XXX_no_io, di) = out(XXX)
	].

:- pred io(pred(io__state, io__state), T, T) <= contains_io_state(T).
:- mode io(pred(di, uo) is det, di(XXX), uo(XXX)) is det.

io(P) -->
	IO0 =^ io_state,
	P(IO0, IO),
	^io_state := IO.

write_strings(Strs) -->
	list__foldl(io(io__write_string), Strs).

Simon.
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