[Haskell-cafe] Fwd: C9 video in the Monadic Design Patterns for the Web series
Greg Meredith
lgreg.meredith at biosimilarity.com
Wed Jul 27 20:12:33 CEST 2011
Dear James,
This is so cool! It's so natural to express this as a monad transformer.
It's great insight and it's just the sort of insight that Haskell and this
way of thinking about computation makes possible. Bravo!
Best wishes,
--greg
On Wed, Jul 27, 2011 at 6:33 AM, James Cook <mokus at deepbondi.net> wrote:
> Dang, I should have played with both versions before sending this. The 'R'
> instance has a very obvious error:
>
> > return x = R (ConwayT (return (Left x)) mzero)
>
> should be changed to
>
> > return x = R (ConwayT mzero (return (Left x)))
>
> Sorry!
>
> -- James
>
> On Jul 27, 2011, at 9:28 AM, James Cook wrote:
>
> For any who are interested, here's a quick and dirty Haskell version of the
> generalized Conway game monad transformer described in the video. It uses
> two newtypes, "L" and "R", to select from two possible implementations of
> the Monad class.
>
> (all the LANGUAGE pragmas are just to support a derived Show instance to
> make it easier to play around with in GHCi - the type and monad itself are
> H98)
>
> -- James
>
>
> > {-# LANGUAGE StandaloneDeriving #-}
> > {-# LANGUAGE FlexibleInstances #-}
> > {-# LANGUAGE UndecidableInstances #-}
> > module Monads.Conway where
> >
> > import Control.Applicative
> > import Control.Monad
> >
> > data ConwayT m a
> > = ConwayT
> > { runLeftConwayT :: m (Either a (ConwayT m a))
> > , runRightConwayT :: m (Either a (ConwayT m a))
> > }
> >
> > deriving instance (Eq a, Eq (m (Either a (ConwayT m a)))) => Eq
> (ConwayT m a)
> > deriving instance (Ord a, Ord (m (Either a (ConwayT m a)))) => Ord
> (ConwayT m a)
> > deriving instance (Read a, Read (m (Either a (ConwayT m a)))) => Read
> (ConwayT m a)
> > deriving instance (Show a, Show (m (Either a (ConwayT m a)))) => Show
> (ConwayT m a)
> >
> > instance Functor m => Functor (ConwayT m) where
> > fmap f (ConwayT l r) = ConwayT (fmap g l) (fmap g r)
> > where
> > g (Left x) = Left (f x)
> > g (Right x) = Right (fmap f x)
> >
> > bind liftS (ConwayT l r) f = ConwayT
> > (liftS g l)
> > (liftS g r)
> > where
> > g (Left x) = Right (f x)
> > g (Right x) = Right (bind liftS x f)
> >
> > newtype L f a = L { runL :: f a } deriving (Eq, Ord, Read, Show)
> >
> > instance Functor m => Functor (L (ConwayT m)) where
> > fmap f (L x) = L (fmap f x)
> >
> > instance MonadPlus m => Monad (L (ConwayT m)) where
> > return x = L (ConwayT (return (Left x)) mzero)
> > L x >>= f = L (bind liftM x (runL . f))
> >
> > newtype R f a = R { runR :: f a } deriving (Eq, Ord, Read, Show)
> >
> > instance Functor m => Functor (R (ConwayT m)) where
> > fmap f (R x) = R (fmap f x)
> >
> > instance MonadPlus m => Monad (R (ConwayT m)) where
> > return x = R (ConwayT (return (Left x)) mzero)
> > R x >>= f = R (bind liftM x (runR . f))
>
>
>
>
> On Jul 27, 2011, at 4:31 AM, Greg Meredith wrote:
>
> Dear Haskellians,
>
> A new C9 video in the series!
>
> So, you folks already know most of this... except for maybe the
> generalization of the Conway construction!
>
> Best wishes,
>
> --greg
>
> ---------- Forwarded message ----------
> From: Charles Torre <...>
> Date: Tue, Jul 26, 2011 at 1:12 PM
> Subject: C9 video in the Monadic Design Patterns for the Web series
> To: Meredith Gregory <lgreg.meredith at gmail.com>
> Cc: Brian Beckman <...>
>
>
> And we’re live!****
>
> ** **
>
>
> http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Greg-Meredith-Monadic-Design-Patterns-for-the-Web-4-of-n
> ****
>
> C****
>
> ** **
>
> *From:* Charles Torre
> *Sent:* Tuesday, July 26, 2011 11:51 AM
> *To:* 'Meredith Gregory'
> *Cc:* Brian Beckman
> *Subject:* C9 video in the Monadic Design Patterns for the Web series****
>
> ** **
>
> Here it ‘tis:****
>
> ** **
>
> Greg Meredith <http://biosimilarity.blogspot.com/>, a mathematician and
> computer scientist, has graciously agreed to do a C9 lecture series covering
> monadic design principles applied to web development. You've met Greg before
> in a Whiteboard jam session with Brian Beckman<http://channel9.msdn.com/shows/Going+Deep/E2E-Whiteboard-Jam-Session-with-Brian-Beckman-Greg-Meredith-Monads-and-Coordinate-Systems/>
> .****
>
> The fundamental concept here is the monad, and Greg has a novel and
> conceptually simplified explanation of what a monad is and why it matters.
> This is a very important and required first step in the series since the
> whole of it is about the application of monadic composition to real world
> web development.****
>
> In *part 4, *Greg primarily focuses on the idea that *a monad is really an
> API* -- it's a view onto the organization of data and control structures,
> not those structures themselves. In OO terms, it's an *interface*. To make
> this point concrete Greg explores one of the simplest possible data
> structures that supports at least two different, yet consistent
> interpretations of the same API. The structure used, Conway's partisan
> games <http://mathworld.wolfram.com/ConwayGame.html>, turned out to be
> tailor-made for this investigation. Not only does this data structure have
> the requisite container-like shape, it provided opportunities to see just
> what's necessary in a container to implement the monadic interface. ** **
>
> Running throughout the presentation is a more general comparison of reuse
> between an OO approach versus a more functional one. When the monadic API is
> "mixed into" the implementing structure we get less reuse than when the
> implementing structure is passed as a type parameter. Finally, doing the
> work put us in a unique position to see not just how to generalize Conway's
> construction, *monadically*, but the underlying pattern which allows the
> generalization to suggest itself.****
>
> See *part 1
> <http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Greg-Meredith-Monadic-Design-Patterns-for-the-Web-Introduction-to-Monads>
> *See *part 2<http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Greg-Meredith-Monadic-Design-Patterns-for-the-Web-2-of-n>
> **
> *See* part 3<http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Greg-Meredith-Monadic-Design-Patterns-for-the-Web-3-of-n>
> *****
>
> **
> --
> L.G. Meredith
> Managing Partner
> Biosimilarity LLC
> 7329 39th Ave SW
> Seattle, WA 98136
>
> +1 206.650.3740
>
> http://biosimilarity.blogspot.com
>
>
>
>
> --
> L.G. Meredith
> Managing Partner
> Biosimilarity LLC
> 1219 NW 83rd St
> Seattle, WA 98117
>
> +1 206.650.3740
>
> http://biosimilarity.blogspot.com
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>
--
L.G. Meredith
Managing Partner
Biosimilarity LLC
1219 NW 83rd St
Seattle, WA 98117
+1 206.650.3740
http://biosimilarity.blogspot.com
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