On Mon, Jul 01, 2013 at 05:07:00PM +0200, Vlatko Basic wrote:
> Hello Cafe!
>
> I had a (simplified) record
>
> data P = P {
> a :: String,
> b :: String,
> c :: IO String
> } deriving (Show, Eq)
>
> but to get automatic deriving of 'Show' and 'Eq' for 'data P' I have
> created 'newtype IOS' and its 'Show' and 'Eq' instances
>
> newtype IOS = IO String
> instance Show (IOS) where show _ = "(IO String) function"
> instance Eq (IOS) where _ == _ = True
An Eq instance for something containing IO is bound to lead to puzzlement
somewhere down the line. I think you're better off defining something like
data P_lesser = P_lesser {
a_lesser :: String,
b_lesser :: String
} deriving (Show, Eq)
to_lesser p = P_lesser (a p) (b p)
and just factoring everything through "to_lesser" when you want to compare
or show.
Tom
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