gistfile1.prolog

% Hey, I gave the things you did a little thought.
% But altered almost everything, that's why I don't think it's an sufficient answer anylonger so I made a gist.

% Firstof: I removed all your string construction with format and use one atom_codes 
% (btw there is a string type in SWI-Prolog for efficient string handling).

% Next, I thought it should be part of the predicate if you end with a newline or had reached the end of file.
% After that, I put some neat little tricks in there to get backtracking behaviour for the get_line/3 - predicate.
% I hope this helps you in any way.

test(InFile) :-
        open(InFile, read, InStrm),
	get_line(InStrm,Line,How),
	format('~s. String: ~s~n',[How,Line]).

% Offers other lines via backtracking 
% (ignore the repeat line and everything after atom_codes if you only want one line)
get_line(InStrm,Line,How):-
	repeat,
	getline_list(InStrm,List,How),
	atom_codes(Line,List),
	(How = eof->!;
	    true).


getline_list(InStrm,Return,How):-
	get_code(InStrm,Code),
	getline_list_helper(InStrm,Code,Return,How).

% getline_list_helper(+Stream,+Character,-Return,?How)
getline_list_helper(_,   C,[],How):-    member((C,How),[(10,nl),(-1,eof)]),!. % terminates here
getline_list_helper(Strm,C ,[C|X],How):-getline_list(Strm,X,How).