prologfoldmeta-predicate

Prolog Implementation of foldl1

I am trying to implement foldl1 in prolog without using build-in foldl predicate.

My code produce Syntax error: Operator expected:

foldl1(_, [E], E).

foldl1(Predicate, [X,Y|Z], Result) :-
  call(Predicate(X), Y, Ans),
  foldl1(Predicate, [Ans|Z], Result).

I expect:

?- foldl1(concat,['a','b','c','d'],X).
X = abcd.

Thank you!

Solution

  • Predicate(X) is indeed invalid syntax as (in standard Prolog), the name of a compound term cannot be a variable. Use instead:

    foldl1(_, [E], E).

foldl1(Predicate, [X,Y|Z], Result) :-
  call(Predicate, X, Y, Ans),
  foldl1(Predicate, [Ans|Z], Result).