{-# LANGUAGE CPP #-} ----------------------------------------------------------------------------- -- | -- Module : Data.Generics.Text -- Copyright : (c) The University of Glasgow, CWI 2001--2003 -- License : BSD-style (see the LICENSE file) -- -- Maintainer : generics@haskell.org -- Stability : experimental -- Portability : non-portable (uses Data.Generics.Basics) -- -- \"Scrap your boilerplate\" --- Generic programming in Haskell -- See <http://www.cs.uu.nl/wiki/GenericProgramming/SYB>. The present module -- provides generic operations for text serialisation of terms. -- ----------------------------------------------------------------------------- module Data.Generics.Text ( -- * Generic show gshow, gshows, -- * Generic read gread ) where ------------------------------------------------------------------------------ #ifdef __HADDOCK__ import Prelude #endif import Control.Monad import Data.Data import Data.Generics.Aliases import Text.ParserCombinators.ReadP ------------------------------------------------------------------------------ -- | Generic show: an alternative to \"deriving Show\" gshow :: Data a => a -> String gshow x = gshows x "" -- | Generic shows gshows :: Data a => a -> ShowS -- This is a prefix-show using surrounding "(" and ")", -- where we recurse into subterms with gmapQ. gshows = ( \t -> showChar '(' . (showString . showConstr . toConstr $ t) . (foldr (.) id . gmapQ ((showChar ' ' .) . gshows) $ t) . showChar ')' ) `extQ` (shows :: String -> ShowS) -- | Generic read: an alternative to \"deriving Read\" gread :: Data a => ReadS a {- This is a read operation which insists on prefix notation. (The Haskell 98 read deals with infix operators subject to associativity and precedence as well.) We use fromConstrM to "parse" the input. To be precise, fromConstrM is used for all types except String. The type-specific case for String uses basic String read. -} gread = readP_to_S gread' where -- Helper for recursive read gread' :: Data a' => ReadP a' gread' = allButString `extR` stringCase where -- A specific case for strings stringCase :: ReadP String stringCase = readS_to_P reads -- Determine result type myDataType = dataTypeOf (getArg allButString) where getArg :: ReadP a'' -> a'' getArg = undefined -- The generic default for gread allButString = do -- Drop " ( " skipSpaces -- Discard leading space _ <- char '(' -- Parse '(' skipSpaces -- Discard following space -- Do the real work str <- parseConstr -- Get a lexeme for the constructor con <- str2con str -- Convert it to a Constr (may fail) x <- fromConstrM gread' con -- Read the children -- Drop " ) " skipSpaces -- Discard leading space _ <- char ')' -- Parse ')' skipSpaces -- Discard following space return x -- Turn string into constructor driven by the requested result type, -- failing in the monad if it isn't a constructor of this data type str2con :: String -> ReadP Constr str2con = maybe mzero return . readConstr myDataType -- Get a Constr's string at the front of an input string parseConstr :: ReadP String parseConstr = string "[]" -- Compound lexeme "[]" <++ string "()" -- singleton "()" <++ infixOp -- Infix operator in parantheses <++ readS_to_P lex -- Ordinary constructors and literals -- Handle infix operators such as (:) infixOp :: ReadP String infixOp = do c1 <- char '(' str <- munch1 (not . (==) ')') c2 <- char ')' return $ [c1] ++ str ++ [c2]