"creates a difference-list" implies:

• creates an open list Codes ending in Tail if Tail can an unbound variable
• creates a closed list if Tail is the empty list
• prefixes a list Tail with the elements read if Tail is a (proper or open) list

attempt(File,Codes,X) :-
    set_prolog_flag(answer_write_options,[max_depth(0)]),
    setup_call_cleanup(
       open(File,read,Stream,[type(text),encoding(utf8)]),
       read_stream_to_codes(Stream,Codes,X),
       (format("Closing",[]),close(Stream))). % N.B: cleanup goal is called only once "call" goal is "done"

?- attempt('small.txt',Codes,X).
Codes = [73,116,39,115,32,115,109,97,108,108,33,10|$VAR(X)].

?- attempt('small.txt',Codes,X),X=[].
Codes = [73,116,39,115,32,115,109,97,108,108,33,10],
X = [].

?- attempt('small.txt',Codes,a).
Codes = [73,116,39,115,32,115,109,97,108,108,33,10|a].

?- attempt('small.txt',Codes,[]).
Codes = [73,116,39,115,32,115,109,97,108,108,33,10].