Re: [Haskell-cafe] "Casting" newtype to base type?

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