the Prolog notation of prefix/suffix is a quite easy one: It pretty much puts all the work on append. For those who don't know:
prefix(P,L):-append(P,_,L).
suffix(S,L):-append(_,S,L).
Now this means, that the result for prefix(X,[a,b,c,d]).
will be: X=[];X=[a];X=[a,b];X=[a,b,c];X=[a,b,c,d]
Here is my problem with this: I want a "real" prefix. Hence, a prefix cannot be empty, nor can the part following it be empty.
So the result to the query prefix(X,[a,b,c,d]). should be
X=[a];X=[a,b];X=[a,b,c]
and that's it.
Unfortunately, the real beaty of the standard-built in prefix predicate is, that it can use the termination of append, which is append([],Y,Y).
So it is pretty easy to know when to stop, picking the list apart one by one till the list is empty.
My termination means: Stop if there is exactly one element left in your list. How do I do this?
My naive result would be:
prefix(P,L):-
length(P,1),append(P,E,L),E/=[].
This feels wrong though. I'm at work so I haven't checked if this actually works, but it should:
Is there any more convenient way to do this? Same goes for suffix, which will be even harder since you do not have a way to adress the Tail as specific as the Head, I guess I'd just reverse the whole thing and then call prefix on it.
Infix will just be a combination of two.
I hope it is clear what I mean. Thanks for your input! tl;dr: How to write a predicate prefix/2 which only filters real prefixes, so the prefix itself can not be empty, nor can the list followed by it be empty.
For the real prefix, you can try to do it like this:
list_prefix(List, [H|T]) :-
append([H|T], [_|_], List).
This just says that the first argument must have at least one element, and the rest of the list must have at least one element.
And following the suggestion by @false to make it more explicit:
list_real_prefix(List, Prefix) :-
Prefix = [_|_],
Rest = [_|_],
append(Prefix, Rest, List).
The "real" suffix will be exactly the same:
list_real_suffix(List, Suffix) :-
Front = [_|_],
Suffix = [_|_],
append(Front, Suffix, List).