Success: concat_strings("abc","def",X). (gives X="abcdef"). [eclipse]: [user]. absolutename(File,Abs) :- string_list(File,List), arg(1,List,0'/) -> Abs = File; (getcwd(Cwd), concat_strings(Cwd,File,Abs)). user compiled 256 bytes in 0.02 seconds yes. [eclipse]: absolutename("d.pl",P), absolutename("/usr/bin",F). P = "/home/lp/user/d.pl" F = "/usr/bin" yes. Fail: concat_strings("ab","bc","abc"). Error: concat_strings("a",X,"ab"). (Error 4). concat_strings("big",'gest',X). (Error 5).