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Editorial Team
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Asked: 2026-06-15T22:44:28+00:00

I am working on writing an insert function that takes two arguments, a number

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I am working on writing an insert function that takes two arguments, a number and a sorted list and it should produce that same list with the number included in its proper position.
Here’s what I got so far: 

insert1(X,[]) :-
   [X].

insert1(X, [H|T]) :-
   X > H,
   insert1(X,T).

insert1(X,[H|_T]) :-
   X < H,
   T is [X|T].

I am getting the following error:

ERROR: '.'/2: Arguments are not sufficiently instantiated ("x" must hold one character)
Exception: (6) insert1(2, [1, 4, 5]) ? creep

I would appreciate your help folks.

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1 Answer

  1. Editorial Team

    The error message it’s rather cryptic: ‘.’/2 it’s the list constructor, and you’re calling it. See this question for a detailed explanation of the syntax.

    Also the last clause is wrong, because is/2 must be used only for arithmetic.

    But generally you are approaching the problem from a wrong ‘perspective’. Arguments in Prolog are immutable. You need another argument to hold the modified list.

    Here is a possible workaround for the first and third clause

    insert1(X, [], [X]).
...
insert1(X, [H|T], [X,H|T]) :-
  X < H.

    You will need to adjust the second clause and invoke insert1 in this way

    ?- insert1(3, [1,2,3,4], L).
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