While that's true, Haskell also makes it easy to make the same sort of error with IO (or any other Monad) values, whether created with the FFI or not. If you say
f = do x y z
and y has type IO CInt then you won't get an error (and I don't think you can even ask for a warning with the current implementations).
Should we have (>>) :: (Monad m) => m () -> m a -> m a and force you to write _ <- y
It's intersting to note that F# follows exactly your proposal. If x has a return type other than () then you do: y |> ignore where ignore :: a -> (), and |> = flip ($) In practice, I found this quite reasonable to use. You also eliminate "errors" such as: do mapM deleteFile files ; return 1 Where mapM requires more memory than the equivalent mapM_ Thanks Neil