[m-dev.] io streams

[David Glen JEFFERY]

On 28-Dec-1999, David Overton <dmo at hydra.cs.mu.oz.au> wrote:
> 
> Perhaps this is one situation where default methods would be nice: the
> default would define read_chars in terms of read_char, but individual
> instances could redefine it if it is possible to write a more
> efficient version.

Yes, but one can achieve roughly the same thing as follows:

:- typeclass stream__input(S) <= stream__stream(S) where
        [
                pred stream__read_char(stream__result(char), S, S),
                mode stream__read_char(out, di, uo) is det,

                pred stream__read_chars(stream__result(list(char)), int, S, S),
                mode stream__read_chars(out, in, di, uo) is det
                
		...
        ].

:- pred default_read_chars(stream__result(list(char)), int, S, S)
	<= stream__input(S).
:- mode default_read_chars(out, di, uo) is det.

default_read_chars(Cs) -->
	read_char(C),
	% etc.

Then, in each instance declaration where you want to use the default, you
can just say:

:- instance stream__input(mytype) where [
	pred(read_char/3) is mytype_read_char,
	pred(read_chars/3) is default_read_chars
].

This is perhaps a minor inconvenience, but also arguably a good way of doing
things.


dgj
-- 
David Jeffery (dgj at cs.mu.oz.au) | If your thesis is utterly vacuous
PhD student,                    | Use first-order predicate calculus.
Dept. of Comp. Sci. & Soft. Eng.|     With sufficient formality
The University of Melbourne     |     The sheerist banality
Australia                       | Will be hailed by the critics: "Miraculous!"
                                |     -- Anon.
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