Verma Anurag wrote:
module Mark where
data Mark = Mark Int deriving (Show)
instance Read Mark where readsPrec _ str = [(Mark x, t') | ("mark",t) <- reads str, (x,t') <- reads t
The problem with this instance is that reads expect Strings to be enclosed in double quotes: *Mark> read "mark" :: String "*** Exception: Prelude.read: no parse *Mark> read "\"mark\"" :: String "mark" Let's try this with your instance: *Mark> read "\"mark\" 4" :: Mark Mark 4 That works, but is probably not what you want. You can use the lex function to parse identifiers not enclosed in quotes:
instance Read Mark where readsPrec _ str = [(Mark x, t') | ("mark",t) <- lex str, (x,t') <- reads t
Now, it's working fine: *Mark> read "mark 4" :: Mark Mark 4 Tillmann