Prolog: Different behaviour of single and double quotes

Question

I'm quite new to Prolog and I stumbled on something that I don't understand.

This is my code:

:- dynamic user/3.
user('id', 'Name', 20).

changeAge(Id, NewAge) :-
   user(Id, Name, _),
   retract(user(Id,_,_)),
   assert(user(Id,Name,NewAge)).

To update user information in the database, changeAge/2 performs these three steps:

1. Lookup a right record, using user/3.
2. Remove one matching record from the database, using retract/1.
3. Insert a new updated record into the database, using assert/1.

This is my console output:

1 ?- user('id', _, Age).
Age = 20.

2 ?- changeAge('id', 25).
true.

3 ?- user('id', _, Age).
Age = 25.

4 ?- changeAge("id", 30).
false.

5 ?- user('id', _, Age).
Age = 25.

Why do single quotes give me true (line 2) when double quotes give me false (line 4)?

Answer 1

TL;DR₁: Read this answer to the question "What is the difference between ' and " in Prolog?".

TL;DR₂: The goal 'id' = "id" succeeds iff the Prolog flag double_quotes is set to atom.

The Prolog flag double_quotes can be set at runtime using set_prolog_flag/2:

• ?- set_prolog_flag(double_quotes, chars).

  ?- 'id' = "id".
  false.
  

• ?- set_prolog_flag(double_quotes, codes).

  ?- 'id' = "id".
  false.
  

• ?- set_prolog_flag(double_quotes, atom).

  ?- 'id' = "id".
  true.
  

For more information read the SICStus Prolog manual page on "Strings as lists"!