[mercury-users] List instantiation question

[Peter Hawkins]

Hi...

On Mar 1, 2006, at 9:51 PM, Ralph Becket wrote:
>
>> Is there some list mode that I am not aware of that solves this  
>> issue?
>
> Not really.
>
> Whenever someone tries to do something Prologesque like this it's
> usually an efficiency hack.

Actually, my personal desire for partial instantiation comes when  
trying to be excessively tricky. My most recent example was when  
trying to write a grammar that can be used for both parsing and  
generation. The idea was something like this (and I thought it was  
one of my more evil ideas for some time =) ):

:- type stream ---> input(...) ; output(...).
:- inst input_stream ---> input(...).
:- inst output_stream ---> output(...).

:- type thing ---> foo(int) ; bar(string).
:- inst partial_thing ---> foo(free) ; bar(free).

:- pred read_or_write_thing(thing, stream, stream).
:- mode read_or_write_thing(out, in(input_stream), out(input_stream))  
is det.
:- mode read_or_write_thing(in, in(output_stream), out 
(output_stream)) is det.

read_or_write_thing(X) -->
	read_or_write_which_of_foo_or_bar(X),
	(
		{ X = foo(A) },
		read_or_write_int(A)
	;
		{ X = bar(B) },
		read_or_write_string(B)
	).

:- pred read_or_write_which_of_foo_or_bar(thing, stream, stream).
:- mode read_or_write_which_of_foo_or_bar(in, in(output_stream), out 
(output_stream)) is det.
:- mode read_or_write_which_of_foo_or_bar(out(partial_thing), in 
(input_stream), out(input_stream)) is det.

Unfortunately you can't actually write the predicate  
read_or_write_which_of_foo_or_bar/3 because it would have to output a  
partially instantiated structure as its first argument in the input  
case.

Ok, maybe this is a gross abuse of the mode system. But it would have  
saved me duplicating my grammar for input and output.

I'll go back to my hole now.

Cheers,
Peter
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