Terminal.hs

{-# OPTIONS_GHC -Wall -Werror #-}

-- | This module handles the complexities of writing information to
-- the terminal, including modifying text in place.  This code is
-- imported from the original HUnit library.
module Test.HUnitPlus.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)