Prolog getting head and tail of string

Question

I\'m trying to wrap my brain around Prolog for the first time (SWI-Prolog) and I\'m struggling with what I\'m sure are the basics. I\'m trying to take a string such as \"pie

Answer 1

The problems with your code are:

spellWord(String) :- String = [H|T], writeChar(H), spellWord(T).

When you give this predicate a long string, it will invoke itself with the tail of that string. But when String is empty, it cannot be split into [H|T], therefore the predicate fails, returning false.

To fix this, you have to define additionally:

spellWord([]).

This is the short form of:

spellWord(String) :- String = [].

Your other predicate also has a problem:

writeChar(String) :- H == "P", print4("Papa").

You have two variables here, String and H. These variables are in no way related. So no matter what you pass as a parameter, it will not influence the H that you use for comparison. And since the == operator only does a comparison, without unification, writeChar fails at this point, returning false. This is the reason why there is no output at all.