String is not string in SWI prolog , part/2.

Last time , this topic was concluded that the string can't
be produced by directly written in the form of "...",
and it should be produced by procedures , eg
string_to_atom/2.

But the problem is not totally solved . Please look :
--------------------------------------------------------------------------------------
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For help, use ?- help(Topic). or ?- apropos(Word).

1 ?- string_to_atom(S,'This is a test').

S = "This is a test"

Yes
2 ?- string_to_atom(S,'This is a test'),
| assertz(db_string(S)),
| tell('c:\\test.txt'),
| listing(db_string/1),
| told.

S = "This is a test"

Yes
3 ?- db_string(S).

S = "This is a test"

Yes
4 ?- db_string(S),
| string_concat(S,S,S2).

S = "This is a test"
S2 = "This is a testThis is a test"

Yes
5 ?- retractall(db_string(_)).

Yes
6 ?- db_string(S).

No
7 ?- consult('c:\\test.txt').
% c:\test.txt compiled 0.00 sec, 520 bytes

Yes
8 ?- string_concat(S,S,S2).
ERROR: string_concat/3: Arguments are not sufficiently instantiated
9 ?- db_string(S),
| string_concat(S,S,S2).
ERROR: string_concat/3: Type error: `atomic' expected, found `[84, 104,
105, 115, 32, 105, 115, 32, 97, 32, 116, 101, 115, 116]'

---------------------------------------------------------------------------
You can see that , there is problem in the step 9 , although the
identical
codes get allright in the step 4.

For more clarification , just look the contents of c:\test.tx:
+++++++++++++++++++++++++++++++++++++++++

:- dynamic db_string/1.

db_string("This is a test").
++++++++++++++++++++++++++++++++++++++++++

It takes the form of db_string/1 with a parameter made of string .
But it just compiles to something that causes string_concat out of
order.
So weird !

Maybe that is because things chages if saved by listing and retrieved
by consult/1. For circumvention from such possibility ,
the following topic happens.

.

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