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
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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