Question

I’m trying to learn prolog and i bumpt in this error which, i don’t know why i get it so i am asking for your help.

code(TPROLOG#86):

trace 

domains
    item = integer
    intList = item*

predicates
    member(item,intList)

clauses
    member(elm,[elm|_]).

    member(elm,[_|T]):-  %%%  ***ELM is seen as wrong type, why?***
        member(item,[T]).

goal
    member(5,[1,2,3,4,5])

Any advice or hint is welcomed. Thank you.

Answer 1

You are confusing variables and atoms. Atoms start with a lower case letter, whereas variables start with an upper case letter.

Also, your member/2 definition seems wrong. It should read:

clauses
    member(Elm,[Elm|_]).
    member(Elm,[_|T]):-  
        member(Elm,T).

First clause matches the element with the head of the second list. Second clause skips the head of the second list and recursively calls member/2 to find another match in the tail of the list.