[Haskell-cafe] How to implement Read instance for user defined type
Anurag.Verma at motorola.com
Thu Mar 20 10:10:11 EDT 2008
I am trying to understand how to define instances for Read class for a
user defined type.
This is the sample code I wrote, but I am not able to get it correctly.
So, let me know what's wrong with this program:
module Mark where
data Mark = Mark Int deriving (Show)
instance Read Mark where
readsPrec _ str = [(Mark x, t') | ("mark",t) <- reads str,
When I run it in ghci:
*Mark> reads "mark 5" :: [ (Mark,String)] ( I am intentionally
using mark in lowercase)
Why is the output coming out as  as against expected output [(Mark 5,
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