    -- | This module handles the complexities of writing information to the
    -- terminal, including modifying text in place.

    module Test.HUnit.Terminal (
            terminalAppearance
        ) where

    import Data.Char (isPrint)

    -- | Simplifies the input string by interpreting @\\r@ and @\\b@ characters
    -- specially so that the result string has the same final (or /terminal/,
    -- pun intended) appearance as would the input string when written to a
    -- terminal that overwrites character positions following carriage
    -- returns and backspaces.

    terminalAppearance :: String -> String
    terminalAppearance str = ta id "" "" str

    -- | The helper function @ta@ takes an accumulating @ShowS@-style function
    -- that holds /committed/ lines of text, a (reversed) list of characters
    -- on the current line /before/ the cursor, a (normal) list of characters
    -- on the current line /after/ the cursor, and the remaining input.

    ta
        :: ([Char] -> t) -- ^ An accumulating @ShowS@-style function
                         -- that holds /committed/ lines of text
        -> [Char] -- ^ A (reversed) list of characters
                  -- on the current line /before/ the cursor
        -> [Char] -- ^ A (normal) list of characters
                  -- on the current line /after/ the cursor
        -> [Char] -- ^ The remaining input
        -> t
    ta f    bs  as ('\n':cs) = ta (\t -> f (reverse bs ++ as ++ '\n' : t)) "" "" cs
    ta f    bs  as ('\r':cs) = ta f "" (reverse bs ++ as) cs
    ta f (b:bs) as ('\b':cs) = ta f bs (b:as) cs
    ta _    ""   _ ('\b': _) = error "'\\b' at beginning of line"
    ta f    bs  as (c:cs)
        | not (isPrint c)    = error "invalid nonprinting character"
        | null as            = ta f (c:bs) ""        cs
        | otherwise          = ta f (c:bs) (tail as) cs
    ta f    bs  as       ""  = f (reverse bs ++ as)