X-Git-Url: http://gitweb.michael.orlitzky.com/?p=dead%2Fharbl.git;a=blobdiff_plain;f=src%2FPretty.hs;fp=src%2FPretty.hs;h=360bd4c1b8a4ded3875972f8a30c195284051c5e;hp=0000000000000000000000000000000000000000;hb=edb0055e306eb6b323bbc75bfb7d15089344af81;hpb=e1060ef815f35309c7ca0800a345d7c54ce346bd

diff --git a/src/Pretty.hs b/src/Pretty.hs
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+{-# LANGUAGE FlexibleInstances #-}
+
+-- | A typeclass for pretty-printing. Types that wish to be
+--   pretty-printed should make themselves an instance of the 'Pretty'
+--   class. The only class function that they need to implement is
+--   'pretty_show', which turns the thing into a string in a nice
+--   way. The 'pretty_print' function then prints the result of
+--   'pretty_show' by default.
+--
+module Pretty
+where
+
+import Text.Parsec ( ParseError )
+
+
+class Pretty a where
+  -- | Obtain a pretty 'String' representation of the given thingy.
+  pretty_show :: a -> String
+
+  -- | Pretty-print the given thingy.
+  pretty_print :: a -> IO ()
+  pretty_print = putStrLn . pretty_show
+
+
+-- | Define a 'Pretty' instance for the result of 'parse'. This lets
+--   us pretty-print the result of a parse attempt without worrying
+--   about whether or not it failed. If the parse failed, you get the
+--   same output that you usually would. Otherwise we pretty-print the
+--   parsed value.
+--
+instance Pretty a => Pretty (Either ParseError a) where
+  pretty_show (Left err) = show err
+  pretty_show (Right v)  = pretty_show v