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Today's Topics:
1. Re: Thinking about monads (Brent Yorgey) (Arthur Chan)
----------------------------------------------------------------------
Message: 1
Date: Tue, 14 Apr 2009 15:08:05 -0700
From: Arthur Chan <[email protected]>
Subject: [Haskell-beginners] Re: Thinking about monads (Brent Yorgey)
To: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="utf-8"
You know, I was wondering... if Monads are a subset of Functors, and
Applicative is a subset of Functors, and Monads are a subset of
Applicative... shouldn't it be possible to tack on the definitions that
automatically derive Functor and Applicative? Isn't it the case that there
is really only one way to define Applicative for a Monad anyway? And isn't
there only one way to define fmap for a Monad that makes sense?
On Tue, Apr 14, 2009 at 2:39 PM, <[email protected]> wrote:
> Send Beginners mailing list submissions to
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>
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> or, via email, send a message with subject or body 'help' to
> [email protected]
>
> You can reach the person managing the list at
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>
> When replying, please edit your Subject line so it is more specific
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>
>
> Today's Topics:
>
> 1. Re: Re: Thinking about monads (Brent Yorgey)
> 2. Re: How would you run a monad within another monad?
> (Arthur Chan)
> 3. Re: How would you run a monad within another monad?
> (Arthur Chan)
> 4. Re: How would you run a monad within another monad?
> (Arthur Chan)
> 5. Re: How would you run a monad within another monad?
> (Jason Dusek)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Tue, 14 Apr 2009 16:42:08 -0400
> From: Brent Yorgey <[email protected]>
> Subject: Re: [Haskell-beginners] Re: Thinking about monads
> To: [email protected]
> Message-ID: <[email protected]>
> Content-Type: text/plain; charset=us-ascii
>
> On Mon, Apr 13, 2009 at 03:11:42PM -0700, Michael Mossey wrote:
> >
> > I know Maybe is both a functor and a monad, and I was thinking: what's
> the
> > difference? They are both wrappers on types. Then I realized, the
> > difference is: they have different class definitions.
>
> In fact, every monad should be a functor, but not every functor is a
> monad. Being a monad is a much stronger condition than being a functor.
>
> >
> > class Functor f where
> > fmap :: (a->b) -> f a -> f b
> >
> > (Note how fussy this definition would be in C++. It would be a kind of
> > template, but would probably look a lot more complex and would require
> > lengthy declarations.)
> >
> > class Monad m where
> > a >>= b :: m a -> (a -> m b) -> m b
>
> Don't forget return :: a -> m a ! That's the other key method in the
> Monad
> class. (There are also >> and 'fail' but those are unimportant---the
> first is just a specialization of >>=, and fail is a hack).
>
> -Brent
>
>
> ------------------------------
>
> Message: 2
> Date: Tue, 14 Apr 2009 14:05:22 -0700
> From: Arthur Chan <[email protected]>
> Subject: Re: [Haskell-beginners] How would you run a monad within
> another monad?
> To: Jason Dusek <[email protected]>
> Cc: [email protected]
> Message-ID:
> <[email protected]>
> Content-Type: text/plain; charset="utf-8"
>
> Here's my contrived example that threw the error.
>
> If you go into ghci, and do a `:t (foo' "blah" myDoohickey)`, you will get
> the type signature "IO ()".
> Doing the same for myOtherDoohickey returns "IO True"
>
> So you would think that you'd be able to uncomment the code that makes IO
> an
> instance of Toplevel. foo' is a function that allows IO to run monadic
> values of type Doohickey. But it doesn't work.
>
>
> ---
>
> import IO
> import Control.Monad.Reader
>
>
> class (Monad n) => Doohickey n where
> putRecord :: String -> n ()
>
> class (Monad m) => Toplevel m where
> foo :: (Doohickey n) => FilePath -> n a -> m a
>
> newtype IOToplevelT a = IOToplevelT { runIOToplevelT :: ReaderT Handle IO a
> } deriving (Monad, MonadReader Handle, MonadIO)
>
> instance Doohickey IOToplevelT where
> putRecord = liftIO . putStrLn
>
> foo' s k = do
> f <- liftIO $ openFile s AppendMode
> runReaderT (runIOToplevelT k) f
>
> --instance Toplevel IO where
> -- foo = foo'
>
> myDoohickey = do
> putRecord "foo"
> putRecord "bar"
>
> myOtherDoohickey = do
> putRecord "hello"
> putRecord "world"
> return True
>
>
> On Mon, Apr 13, 2009 at 7:55 PM, Jason Dusek <[email protected]>
> wrote:
>
> > Copypasting and loading your code doesn't throw an error. Please,
> > pastebin an example that demonstrates the error.
> >
> > --
> > Jason Dusek
> >
> -------------- next part --------------
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>
> ------------------------------
>
> Message: 3
> Date: Tue, 14 Apr 2009 14:39:56 -0700
> From: Arthur Chan <[email protected]>
> Subject: Re: [Haskell-beginners] How would you run a monad within
> another monad?
> To: Jason Dusek <[email protected]>
> Cc: [email protected]
> Message-ID:
> <[email protected]>
> Content-Type: text/plain; charset="utf-8"
>
> I seem to have finally solved my own problem, via something I learned from
> RWH. The solution is to use functional dependencies...
>
> The problem was that the compiler needed to know the relationship between
> Doohickeys and Toplevels, and I couldn't figure out how to tell it that...
>
>
>
> {-# LANGUAGE GeneralizedNewtypeDeriving, NoMonomorphismRestriction,
> FunctionalDependencies, MultiParamTypeClasses #-}
>
> import IO
> import Control.Monad.Reader
>
>
> class (Monad n) => Doohickey n where
> putRecord :: String -> n ()
>
> class (Monad m, Doohickey n) => Toplevel m n | m -> n where
> foo :: FilePath -> n a -> m a
>
> newtype IODoohickey a = IODoohickey { runIODoohickey :: ReaderT Handle IO a
> } deriving (Monad, MonadReader Handle, MonadIO)
>
> instance Doohickey IODoohickey where
> putRecord = liftIO . putStrLn
>
> instance Toplevel IO IODoohickey where
> foo s k = do
> f <- liftIO $ openFile s AppendMode
> runReaderT (runIODoohickey k) f
>
>
> myDoohickey = do
> putRecord "foo"
> putRecord "bar"
>
> myOtherDoohickey = do
> putRecord "hello"
> putRecord "world"
> return True
>
>
>
>
> On Tue, Apr 14, 2009 at 2:05 PM, Arthur Chan <[email protected]
> >wrote:
>
> > Here's my contrived example that threw the error.
> >
> > If you go into ghci, and do a `:t (foo' "blah" myDoohickey)`, you will
> get
> > the type signature "IO ()".
> > Doing the same for myOtherDoohickey returns "IO True"
> >
> > So you would think that you'd be able to uncomment the code that makes IO
> > an instance of Toplevel. foo' is a function that allows IO to run
> monadic
> > values of type Doohickey. But it doesn't work.
> >
> >
> > ---
> >
> > import IO
> > import Control.Monad.Reader
> >
> >
> > class (Monad n) => Doohickey n where
> > putRecord :: String -> n ()
> >
> > class (Monad m) => Toplevel m where
> > foo :: (Doohickey n) => FilePath -> n a -> m a
> >
> > newtype IOToplevelT a = IOToplevelT { runIOToplevelT :: ReaderT Handle IO
> a
> > } deriving (Monad, MonadReader Handle, MonadIO)
> >
> > instance Doohickey IOToplevelT where
> > putRecord = liftIO . putStrLn
> >
> > foo' s k = do
> > f <- liftIO $ openFile s AppendMode
> > runReaderT (runIOToplevelT k) f
> >
> > --instance Toplevel IO where
> > -- foo = foo'
> >
> > myDoohickey = do
> > putRecord "foo"
> > putRecord "bar"
> >
> > myOtherDoohickey = do
> > putRecord "hello"
> > putRecord "world"
> > return True
> >
> >
> >
> > On Mon, Apr 13, 2009 at 7:55 PM, Jason Dusek <[email protected]
> >wrote:
> >
> >> Copypasting and loading your code doesn't throw an error. Please,
> >> pastebin an example that demonstrates the error.
> >>
> >> --
> >> Jason Dusek
> >>
> >
> >
> -------------- next part --------------
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>
> ------------------------------
>
> Message: 4
> Date: Tue, 14 Apr 2009 14:40:25 -0700
> From: Arthur Chan <[email protected]>
> Subject: Re: [Haskell-beginners] How would you run a monad within
> another monad?
> To: Jason Dusek <[email protected]>
> Cc: [email protected]
> Message-ID:
> <[email protected]>
> Content-Type: text/plain; charset="utf-8"
>
> I wonder how you would do this with type familes...
>
>
>
> On Tue, Apr 14, 2009 at 2:39 PM, Arthur Chan <[email protected]
> >wrote:
>
> > I seem to have finally solved my own problem, via something I learned
> from
> > RWH. The solution is to use functional dependencies...
> >
> > The problem was that the compiler needed to know the relationship between
> > Doohickeys and Toplevels, and I couldn't figure out how to tell it
> that...
> >
> >
> >
> > {-# LANGUAGE GeneralizedNewtypeDeriving, NoMonomorphismRestriction,
> > FunctionalDependencies, MultiParamTypeClasses #-}
> >
> > import IO
> > import Control.Monad.Reader
> >
> >
> > class (Monad n) => Doohickey n where
> > putRecord :: String -> n ()
> >
> > class (Monad m, Doohickey n) => Toplevel m n | m -> n where
> > foo :: FilePath -> n a -> m a
> >
> > newtype IODoohickey a = IODoohickey { runIODoohickey :: ReaderT Handle IO
> a
> > } deriving (Monad, MonadReader Handle, MonadIO)
> >
> > instance Doohickey IODoohickey where
> > putRecord = liftIO . putStrLn
> >
> > instance Toplevel IO IODoohickey where
> > foo s k = do
> > f <- liftIO $ openFile s AppendMode
> > runReaderT (runIODoohickey k) f
> >
> >
> > myDoohickey = do
> > putRecord "foo"
> > putRecord "bar"
> >
> > myOtherDoohickey = do
> > putRecord "hello"
> > putRecord "world"
> > return True
> >
> >
> >
> >
> > On Tue, Apr 14, 2009 at 2:05 PM, Arthur Chan <[email protected]
> >wrote:
> >
> >> Here's my contrived example that threw the error.
> >>
> >> If you go into ghci, and do a `:t (foo' "blah" myDoohickey)`, you will
> get
> >> the type signature "IO ()".
> >> Doing the same for myOtherDoohickey returns "IO True"
> >>
> >> So you would think that you'd be able to uncomment the code that makes
> IO
> >> an instance of Toplevel. foo' is a function that allows IO to run
> monadic
> >> values of type Doohickey. But it doesn't work.
> >>
> >>
> >> ---
> >>
> >> import IO
> >> import Control.Monad.Reader
> >>
> >>
> >> class (Monad n) => Doohickey n where
> >> putRecord :: String -> n ()
> >>
> >> class (Monad m) => Toplevel m where
> >> foo :: (Doohickey n) => FilePath -> n a -> m a
> >>
> >> newtype IOToplevelT a = IOToplevelT { runIOToplevelT :: ReaderT Handle
> IO
> >> a } deriving (Monad, MonadReader Handle, MonadIO)
> >>
> >> instance Doohickey IOToplevelT where
> >> putRecord = liftIO . putStrLn
> >>
> >> foo' s k = do
> >> f <- liftIO $ openFile s AppendMode
> >> runReaderT (runIOToplevelT k) f
> >>
> >> --instance Toplevel IO where
> >> -- foo = foo'
> >>
> >> myDoohickey = do
> >> putRecord "foo"
> >> putRecord "bar"
> >>
> >> myOtherDoohickey = do
> >> putRecord "hello"
> >> putRecord "world"
> >> return True
> >>
> >>
> >>
> >> On Mon, Apr 13, 2009 at 7:55 PM, Jason Dusek <[email protected]
> >wrote:
> >>
> >>> Copypasting and loading your code doesn't throw an error. Please,
> >>> pastebin an example that demonstrates the error.
> >>>
> >>> --
> >>> Jason Dusek
> >>>
> >>
> >>
> >
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> ------------------------------
>
> Message: 5
> Date: Tue, 14 Apr 2009 14:52:29 -0700
> From: Jason Dusek <[email protected]>
> Subject: Re: [Haskell-beginners] How would you run a monad within
> another monad?
> To: Arthur Chan <[email protected]>
> Cc: [email protected]
> Message-ID:
> <[email protected]>
> Content-Type: text/plain; charset=UTF-8
>
> Congratulations.
>
> --
> Jason Dusek
>
>
> ------------------------------
>
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