[mercury-users] $pred doesn't work in lambda expressions

[Peter Ross]

On Thu, Jul 24, 2008 at 10:55 AM, Peter Wang <novalazy at gmail.com> wrote:
> On 2008-06-26, Peter Wang <novalazy at gmail.com> wrote:
>> On 2008-06-26, Nicholas Nethercote <njn at csse.unimelb.edu.au> wrote:
>> >
>> > Hi,
>> >
>> > I tried using $pred in a lambda expression and got this:
>> >
>> >   Uncaught Mercury exception:
>> >   Software Error: hlds_code_util.m: Unexpected: cons_id_to_tag: implementation_defined_const
>> >
>> > It's easy to work around, but I thought it might be worth mentioning.
>>
>> Thanks for pointing it out.  Added to the bug tracker.
>
> This was fixed about a month ago.  However,
>
>    main(!IO) :-
>        P = (pred(!.IO::di, !:IO::uo) is det :-
>            io.write_string($pred, !IO),
>            io.nl(!IO)
>        ),
>        P(!IO).
>
> will print
>
>    predicate `pre.main'/2
>
> which, while correct by one view, seems counter-intuitive.
> Any suggestions on how to expand $pred in lambdas?
>
I would suggest "lambda in pre.main/2 on line xx"
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