[Haskell-cafe] How to implement Read instance for user defined type

[Tillmann Rendel]

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