Is there a nicer way to extract the 'IO String' from 'IOS',
without 'case' or without pattern matching the whole 'P'?
newtype IOS = IOS (IO String)
data P = P {
getA :: String,
getB :: String,
getC :: IOS
} deriving (Show, Eq)
getC_IO :: P -> IO String
getC_IO p =
case getC p of
IOS a -> a
getC_IO (P _ _ (IOS a)) = a
-------- Original Message --------
Subject: Re: [Haskell-cafe] "Casting" newtype to base type?
From: Malcolm Wallace <malcolm.wall...@me.com>
To: vlatko.ba...@gmail.com
Cc: Haskell-Cafe <haskell-cafe@haskell.org>
Date: 01.07.2013 17:24
On 1 Jul 2013, at 16:07, Vlatko Basic wrote:
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
Not quite! That is a newtype'd String, not a newtype's (IO String). Try this:
newtype IOS = IOS (IO String)
but now when I try to set 'c' field in
return $ p {c = readFile path}
I get error
Couldn't match expected type `IOS' with actual type `IO String'
Use the newtype constructor to convert an IO String -> IOS.
return $ p {c = IOS $ readFile path}
Regards,
Malcolm
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