[Haskell-cafe] Re: Monad transformer, liftIO

[Paolo Losi]

Michael Roth wrote:
> Hello list,
> 
> maybe I'm just stupid, I'm trying to do something like this:

Ciao Michael,

As an alternative solution to Creighton's:

     import Control.Monad.List

     foobar :: ListT IO Int
     foobar = do
       a <- ListT . return $ [1,2,3]
       b <- ListT . return $ [4,5,6]
       liftIO $ putStrLn $ (show a) ++ " " ++ (show b)
       return (a+b)

     main = do
       sums <- runListT foobar
       print sums


For the expression ListT . return $ [1,2,3] of type ListT IO Int,
the return inferred is:

return: a -> IO a
return [1,2,3] = IO [1,2,3]

and then you wrap the IO [] with the ListT newtype constructor.

Paolo