A-M baby

Task/Input-loop/Mercury/input-loop.mercury

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+:- module input_loop.
+:- interface.
+
+:- import_module io.
+:- pred main(io::di, io::uo) is det.
+
+:- implementation.
+
+main(!IO) :-
+    io.stdin_stream(Stdin, !IO),
+    io.stdout_stream(Stdout, !IO),
+    read_and_print_lines(Stdin, Stdout, !IO).
+
+:- pred read_and_print_lines(io.text_input_stream::in,
+    io.text_output_stream::in, io::di, io::uo) is det.
+
+read_and_print_lines(InFile, OutFile, !IO) :-
+    io.read_line_as_string(InFile, Result, !IO),
+    (
+        Result = ok(Line),
+        io.write_string(OutFile, Line, !IO),
+        read_and_print_lines(InFile, OutFile, !IO)
+    ;
+        Result = eof
+    ;
+        Result = error(IOError),
+        Msg = io.error_message(IOError),
+        io.stderr_stream(Stderr, !IO),
+        io.write_string(Stderr, Msg, !IO),
+        io.set_exit_status(1, !IO)
+    ).