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 - ra...@froglogic.com
www.froglogic.com - Multi-Platform GUI Testing
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
