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
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