Re: [Haskell-cafe] ANNOUNCE: monad-control-0.3

6 Dec 2011


      On Tue, Dec 6, 2011 at 3:03 PM, Bas van Dijk  wrote:
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On 6 December 2011 12:59, Michael Snoyman  wrote:
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On Tue, Dec 6, 2011 at 11:49 AM, Bas van Dijk  wrote:
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On 6 December 2011 05:06, Michael Snoyman  wrote:
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Maybe this will help[1]. It's using RWST instead of StateT, but it's
the same idea.
[1] https://github.com/yesodweb/yesod/commit/7619e4e9dd88c152d1e00b6fea073c3d52d...
Hi Michael,
Note that you can just reuse the MonadTransControl instance of the
RWST transformer:
instance MonadTransControl (GGWidget master) where
   newtype StT (GGWidget master) a =
       StWidget {unStWidget :: StT (GWInner master) a}
   liftWith f = GWidget $ liftWith $ \run ->
                  f $ liftM StWidget . run . unGWidget
   restoreT = GWidget . restoreT . liftM unStWidget
Cheers,
Bas
Thanks Bas, I was just in the process of converting Widget from being
a RWS to a Writer, and your code made it much simpler :).
Michael
Do you think it's useful to have the following two utility functions
for defining a MonadTransControl instance for your own monad
transformer provided that your transformers is defined in terms of
another transformer:
defaultLiftWith ∷ (Monad m, MonadTransControl tInner)
               ⇒ (tInner m α → t m α)          -- ^ Constructor
               → (∀ β n. t n β → tInner n β)   -- ^ Deconstructor
               → (∀ β. StT tInner β → StT t β) -- ^ State constructor
               → ((Run t → m α) → t m α)
defaultLiftWith con deCon st = \f → con $ liftWith $ \run →
                                     f $ liftM st ∘ run ∘ deCon
defaultRestoreT ∷ (Monad m, MonadTransControl tInner)
               ⇒ (tInner m α → t m α)      -- ^ Constructor
               → (StT t α  → StT tInner α) -- ^ State deconstructor
               → (m (StT t α) → t m α)
defaultRestoreT con unSt = con ∘ restoreT ∘ liftM unSt
For example in your case you would use these as follows:
instance MonadTransControl (GGWidget master) where
   newtype StT (GGWidget master) a =
       StWidget {unStWidget :: StT (GWInner master) a}
   liftWith = defaultLiftWith GWidget unGWidget StWidget
   restoreT = defaultRestoreT GWidget unStWidget
Bas
I don't have a strong opinion, but it sounds like a net win, assuming
the documentation clearly explains how they are supposed to be used.

Michael