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module Main (main) where
import Data.Functor.Classes
import Data.Functor.Compose
import Text.ParserCombinators.ReadP as P
import Text.ParserCombinators.ReadPrec (ReadPrec, lift, minPrec, readPrec_to_S)
readEither' :: ReadPrec a -> String -> Either String a
readEither' rp s =
case [ x | (x,"") <- readPrec_to_S read' minPrec s ] of
[x] -> Right x
[] -> Left "read1: no parse"
_ -> Left "read1: ambiguous parse"
where
read' =
do x <- rp
lift P.skipSpaces
return x
-- | Like 'read', but tailored to 'Read1'.
read1 :: (Read1 f, Read a) => String -> f a
read1 s = either errorWithoutStackTrace id (readEither' readPrec1 s)
exRead, exRead1 :: Compose Maybe Maybe Int
exRead = read "Compose Nothing"
exRead1 = read1 "Compose Nothing"
main :: IO ()
main = do
putStrLn $ showsPrec 0 exRead ""
putStrLn $ showsPrec1 0 exRead1 ""
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