[Haskell-cafe] more gtk help

Brandon Allbery allbery.b at gmail.com
Wed Aug 14 07:17:39 CEST 2013

On Tue, Aug 13, 2013 at 10:45 PM, <briand at 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 at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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