On Tue, Aug 13, 2013 at 10:45 PM, <briand@aracnet.com> wrote:

fooBar =
do putStrLn "foo"
return True

so then I thought, aha!, all I need to do is understand the type of "return True" and all will be revealed to me. Well, it's this:

Control.Monad.Trans.Reader.ReaderT
(GHC.Ptr.Ptr Gtk.EExpose) IO Bool

just like the error message says.

Still don't know what that's supposed to be. I'm having trouble tracking down

Control.Monad.Trans.Reader.ReaderT

In this case, all you need to know is the Control.Monad.Trans part and the IO underneath; this tells you that you can use `lift` and possibly `liftIO` to get at the IO.

fooBar = do

liftIO $ putStrLn "foo"

return True

If `liftIO` complains about a missing MonadIO instance, file a bug :) but you can also get there by using `lift` to reach it; in this case you only need it once, but for more deeply nested transformers you may need it multiple times (e.g. `lift . lift . lift $ putStrLn "foo"` for a stack of 3 transformers over IO).

brandon s allbery kf8nh sine nomine associates

allbery.b@gmail.com ballbery@sinenomine.net

unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net