src/Pretty.hs

   1 {-# LANGUAGE FlexibleInstances #-}
   2
   3 -- | A typeclass for pretty-printing. Types that wish to be
   4 --   pretty-printed should make themselves an instance of the 'Pretty'
   5 --   class. The only class function that they need to implement is
   6 --   'pretty_show', which turns the thing into a string in a nice
   7 --   way. The 'pretty_print' function then prints the result of
   8 --   'pretty_show' by default.
   9 --
  10 module Pretty ( Pretty(..) )
  11 where
  12
  13 import Text.Parsec ( ParseError )
  14
  15
  16 class Pretty a where
  17   -- | Obtain a pretty 'String' representation of the given thingy.
  18   pretty_show :: a -> String
  19
  20   -- | Pretty-print the given thingy.
  21   pretty_print :: a -> IO ()
  22   pretty_print = putStrLn . pretty_show
  23
  24
  25 -- | If we can pretty print something, we can pretty-print a list of
  26 --   them too.
  27 --
  28 instance (Pretty a) => Pretty [a] where
  29   pretty_show l = show $ map pretty_show l
  30
  31
  32 -- | If we can pretty print something, we can pretty-print a pair of
  33 --   them too.
  34 --
  35 instance (Pretty a, Pretty b) => Pretty (a,b) where
  36   pretty_show (x,y) = show (pretty_show x, pretty_show y)
  37
  38
  39
  40 -- | Define a 'Pretty' instance for the result of 'parse'. This lets
  41 --   us pretty-print the result of a parse attempt without worrying
  42 --   about whether or not it failed. If the parse failed, you get the
  43 --   same output that you usually would. Otherwise we pretty-print the
  44 --   parsed value.
  45 --
  46 instance Pretty a => Pretty (Either ParseError a) where
  47   pretty_show (Left err) = show err
  48   pretty_show (Right v)  = pretty_show v