[mercury-users] full and empty lists

[Dave Slutzkin]

Hi all.

I have a type 'timeline' (which should probably be a
typeclass - at the moment it isn't), implemented using
the standard 'list' module.  But I've tried to put
wrappers around all of the functions that access it so
I can change it easily later.  I have a snippet of
code that looks like this:

:- mode ne_tl_io == ground >> non_empty_list.
:- mode ne_tl_in == non_empty_list >> non_empty_list.

:- func empty_timeline = (timeline::out) is det.
:- pred full_timeline(timeline::ne_tl_io) is semidet.

empty_timeline = [].
full_timeline([_|_]).

Bascially - I was hoping that empty_timeline and
full_timeline could be used in a switch statement the
same way as [] and [_|_].

Also, after a call to full_timeline, I was hoping that
the compiler would realise that I had a non-empty list
and so I could save this checking in many later
predicates.  Another snippet:

:- pred crop_timeline(timeline::in,time::in,
timeline::out) is det.
crop_timeline(Line,T,NewLine) :-
    Line = empty_timeline ,
    NewLine = empty_timeline
;
    full_timeline(Line) ,
    % we know we have a non-empty list here
    E = first_event(Line) ,
.
.
.

Note the use of empty_timeline and full_timeline in
the switch (are they guards?).  And also that
first_event can be deterministic, because it takes a
non-empty list so will always succeed.

Well, it doesn't work.  The compiler still sees this
code as nondeterministic - I quote:

event.m:108:   In the return value of call to function
`event:empty_timeline/0':
event.m:108:   unification with `Line' can fail.
event.m:111:   call to
`full_timeline((event:ne_tl_io))' can fail.

However, it still works when I replace empty_timeline
with [] and full_timeline with [_|_] - although
without the mode benefits of non_empty_list.

Is there anything I can do about this?  Does anyone
even understand the problem?  Otherwise I'll have to
go back to the less modular, less deterministic
version.

thanks,

Dave.

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