Preserve comments from quoted source.

src/merl.erl

@@ -219,11 +219,9 @@ quote_1(StartLine, StartCol, Text) ->
                    _ when StartCol =:= undefined -> StartLine;
                    _ -> {StartLine, StartCol}
                end,
-    {ok, Ts, _} = erl_scan:string(flatten_text(Text), StartPos),
-    case parse_1(Ts) of
-        [T] -> T;
-        Other -> Other
-    end.
+    FlatText = flatten_text(Text),
+    {ok, Ts, _} = erl_scan:string(FlatText, StartPos),
+    merge_comments(StartLine, erl_comment_scan:string(FlatText), parse_1(Ts)).
 
 parse_1(Ts) ->
     %% if dot tokens are present, it is assumed that the text represents
@@ -916,3 +914,35 @@ make_tree(record_field, [[N], []]) -> erl_syntax:record_field(N);
 make_tree(record_field, [[N], [E]]) -> erl_syntax:record_field(N, E);
 make_tree(Type, Groups) ->
     erl_syntax:make_tree(Type, Groups).
+
+merge_comments(_StartLine, [], [T]) -> T;
+merge_comments(_StartLine, [], Ts) -> Ts;
+merge_comments(StartLine, Comments, Ts) ->
+    merge_comments(StartLine, Comments, Ts, []).
+
+merge_comments(_StartLine, [], [], [T]) -> T;
+merge_comments(_StartLine, [], [T], []) -> T;
+merge_comments(_StartLine, [], Ts, Acc) ->
+    lists:reverse(Acc, Ts);
+merge_comments(StartLine, Cs, [], Acc) ->
+    merge_comments(StartLine, [], [],
+                   [erl_syntax:set_pos(
+                      erl_syntax:comment(Indent, Text),
+                      StartLine + Line - 1)
+                    || {Line, _, Indent, Text} <- Cs] ++ Acc);
+merge_comments(StartLine, [C|Cs], [T|Ts], Acc) ->
+    {Line, _Col, Indent, Text} = C,
+    CommentLine = StartLine + Line - 1,
+    case erl_syntax:get_pos(T) of
+        Pos when Pos < CommentLine ->
+            %% TODO: traverse sub-tree rather than only the top level nodes
+            merge_comments(StartLine, [C|Cs], Ts, [T|Acc]);
+        CommentLine ->
+            Tc = erl_syntax:add_postcomments(
+                   [erl_syntax:comment(Indent, Text)], T),
+            merge_comments(StartLine, Cs, [Tc|Ts], Acc);
+        _ ->
+            Tc = erl_syntax:add_precomments(
+                   [erl_syntax:comment(Indent, Text)], T),
+            merge_comments(StartLine, Cs, [Tc|Ts], Acc)
+    end.

src/merl_tests.erl

@@ -124,6 +124,19 @@ quote_case_clause_test_() ->
                          "{X, Y} when X > Y -> 1;"
                          "_ -> 0"])))].
 
+quote_comment_test_() ->
+    [?_assertEqual("%% comment preserved\n"
+                   "{foo, 42}",
+                  f(?Q(["%% comment preserved",
+                        "{foo, 42}"]))),
+     ?_assertEqual("{foo, 42}"
+                   "%% comment preserved\n",
+                   f(?Q(["{foo, 42}",
+                         "%% comment preserved"]))),
+     ?_assertEqual("  % just a comment (with indent)\n",
+                  f(?Q("  % just a comment (with indent)")))
+    ].
+
 metavar_test_() ->
     [?_assertEqual("'@foo'", f(merl:tree(merl:template(?Q("'@foo'"))))),
      ?_assertEqual("'@foo'", f(merl:tree(merl:template(?Q("_@foo"))))),