This is the core functionality from the indent parser (sequential Erlang version)

parse_indents.erl

-record(line, {content, indent}).
-record(node, {content, children}).

parse_text_line(Text) ->
    %% converts "    text" to (:content "text" :indent 4)
    Content = string:strip(string:strip(Text, left), left, $\t),
    Indent = length(Text) - length(Content),
    #line{content=string:strip(Content, right, $\n), indent=Indent}.

%% Split a list of lines in two, breaking on the first line whose indent is =< the minimum.
split_list(_MinIndent, List1, []) -> [lists:reverse(List1), []];
split_list(MinIndent, List1, [First|Rest]) when First#line.indent =< MinIndent ->
    [lists:reverse(List1), [First|Rest]];
split_list(MinIndent, List1, [First|Rest]) ->
    split_list(MinIndent, [First|List1], Rest).

%% Parse a list of 'line' record into a tree structure, based on indent
build_nodes([]) -> [];
build_nodes([First|Rest]) ->
    [ChildLines, SiblingLines] = split_list(First#line.indent, [], Rest),
    Children = build_nodes(ChildLines),
    Siblings = build_nodes(SiblingLines),
    Node = #node{content=First#line.content, children=Children},
    [Node|Siblings].

I eventually discovered lists:splitwith/2, which obsolesces my split_list/3. So you can throw that out and replace line 20 with:

IsChild = fun(L) -> L#line.indent > First#line.indent end,
{ChildLines, SiblingLines} = lists:splitwith(IsChild, Rest),

Here's the full code for the sequential Erlang version of the indent parser. There's also a concurrent Erlang version and a Lisp version.