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
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