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

[Frerich Raabe]

Am 7/1/2013 5:07 PM, schrieb Vlatko Basic:
> 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

What you really want is

   newtype IOS = IOS (IO String)

I.e. a IOS value "wraps" an IO String.

>    data P = P {
>      a :: String,
>      b :: String,
>      c :: IOS
>      } deriving (Show, Eq)
>
> but now when I try to set 'c' field in
>
>    fun :: FilePath -> P -> IO P
>    fun path p = do
>      b <- doesFileExist path
>      ...
>      return $ p {c = readFile path}
>
> I get error
>    Couldn't match expected type `IOS' with actual type `IO String'
>
> which is correct.
>
> So, the question is:
>
>    Is it possible to somehow "cast" 'IO String' to 'IOS'

With the change to your 'newtype', you could use

   return $ p {c = IOS (readFile path)}

instead.

-- 
Frerich Raabe - raabe at froglogic.com
www.froglogic.com - Multi-Platform GUI Testing