"key[subkey][subsubkey]" => "some_value"

    {"key" => {"subkey" => {"subsubkey" => "some_value"}}}

Valid keys would be:

    "key"
    "key[subkey]"
    "key[subkey][subsubkey]"
    etc.

    let Right x = paramTree [("a[x]","b"),("a[b]","c"),
    ("a[b][c]","d"),("bc[de]","f")] in putStrLn (drawTree x)

    { "a" => { "b" => <conflict>, "x" => "b" }, "bc" => {
    "de" => "f" } }

yc.hs:35:34: No instance for (Monad (Either String)) arising from a use of `mapM' at yc.hs:35:34-52 Possible fix: add an instance declaration for (Monad (Either String)) In a 'do' expression: keys <- mapM parseKey names In the expression: do keys <- mapM parseKey names return (toTree (zip keys values)) In the definition of `paramTree': paramTree nameValues = do keys <- mapM parseKey names return (toTree (zip keys values)) where (names, values) = unzip nameValues

(Edit: I'm sorry that the error message's whitespace gets messed up here.)

Also your groupings function is only used in conjunction with Data.Map.fromList. Did you consider using something like Data.Map.fromListWith (++) instead? Of course you'd have to wrap your would-be Map values in singleton lists first.

    instance Error e => Monad (Either e) where
        ...

$parsed = str_replace('{', '{"', '{'.str_replace(Array('[', ']'), Array('" => {', ''), $string)) .str_repeat('}', substr_count($string, '[')+1);