I'm experimenting. The IO field is just a helper field, so it shouldn't have any
consequences.
-------- Original Message --------
Subject: Re: [Haskell-cafe] "Casting" newtype to base type?
From: Tom Ellis <tom-lists-haskell-cafe-2...@jaguarpaw.co.uk>
To: haskell-cafe@haskell.org
Date: 01.07.2013 17:24
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
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
