[Haskell-beginners] Re: Thinking about monads (Brent Yorgey)
Arthur Chan
baguasquirrel at gmail.com
Tue Apr 14 18:08:05 EDT 2009
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, <beginners-request at haskell.org> wrote:
> Send Beginners mailing list submissions to
> beginners at haskell.org
>
> To subscribe or unsubscribe via the World Wide Web, visit
> http://www.haskell.org/mailman/listinfo/beginners
> or, via email, send a message with subject or body 'help' to
> beginners-request at haskell.org
>
> You can reach the person managing the list at
> beginners-owner at haskell.org
>
> When replying, please edit your Subject line so it is more specific
> than "Re: Contents of Beginners digest..."
>
>
> 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 <byorgey at seas.upenn.edu>
> Subject: Re: [Haskell-beginners] Re: Thinking about monads
> To: beginners at haskell.org
> Message-ID: <20090414204207.GA16671 at seas.upenn.edu>
> 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 <baguasquirrel at gmail.com>
> Subject: Re: [Haskell-beginners] How would you run a monad within
> another monad?
> To: Jason Dusek <jason.dusek at gmail.com>
> Cc: beginners at haskell.org
> Message-ID:
> <74cabd9e0904141405i1fbadb85u8b87ffb05d61c493 at mail.gmail.com>
> 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 <jason.dusek at gmail.com>
> wrote:
>
> > Copypasting and loading your code doesn't throw an error. Please,
> > pastebin an example that demonstrates the error.
> >
> > --
> > Jason Dusek
> >
> -------------- next part --------------
> An HTML attachment was scrubbed...
> URL:
> http://www.haskell.org/pipermail/beginners/attachments/20090414/0c3d6d92/attachment-0001.htm
>
> ------------------------------
>
> Message: 3
> Date: Tue, 14 Apr 2009 14:39:56 -0700
> From: Arthur Chan <baguasquirrel at gmail.com>
> Subject: Re: [Haskell-beginners] How would you run a monad within
> another monad?
> To: Jason Dusek <jason.dusek at gmail.com>
> Cc: beginners at haskell.org
> Message-ID:
> <74cabd9e0904141439u6b146b78n727cc83b0c16a7ca at mail.gmail.com>
> 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 <baguasquirrel at gmail.com
> >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 <jason.dusek at gmail.com
> >wrote:
> >
> >> Copypasting and loading your code doesn't throw an error. Please,
> >> pastebin an example that demonstrates the error.
> >>
> >> --
> >> Jason Dusek
> >>
> >
> >
> -------------- next part --------------
> An HTML attachment was scrubbed...
> URL:
> http://www.haskell.org/pipermail/beginners/attachments/20090414/c3fd4db2/attachment-0001.htm
>
> ------------------------------
>
> Message: 4
> Date: Tue, 14 Apr 2009 14:40:25 -0700
> From: Arthur Chan <baguasquirrel at gmail.com>
> Subject: Re: [Haskell-beginners] How would you run a monad within
> another monad?
> To: Jason Dusek <jason.dusek at gmail.com>
> Cc: beginners at haskell.org
> Message-ID:
> <74cabd9e0904141440p3ef435c1mba2e7323b196efcf at mail.gmail.com>
> 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 <baguasquirrel at gmail.com
> >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 <baguasquirrel at gmail.com
> >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 <jason.dusek at gmail.com
> >wrote:
> >>
> >>> Copypasting and loading your code doesn't throw an error. Please,
> >>> pastebin an example that demonstrates the error.
> >>>
> >>> --
> >>> Jason Dusek
> >>>
> >>
> >>
> >
> -------------- next part --------------
> An HTML attachment was scrubbed...
> URL:
> http://www.haskell.org/pipermail/beginners/attachments/20090414/e56db9ad/attachment-0001.htm
>
> ------------------------------
>
> Message: 5
> Date: Tue, 14 Apr 2009 14:52:29 -0700
> From: Jason Dusek <jason.dusek at gmail.com>
> Subject: Re: [Haskell-beginners] How would you run a monad within
> another monad?
> To: Arthur Chan <baguasquirrel at gmail.com>
> Cc: beginners at haskell.org
> Message-ID:
> <42784f260904141452j28f12881re823d3b6e3a4d1a5 at mail.gmail.com>
> Content-Type: text/plain; charset=UTF-8
>
> Congratulations.
>
> --
> Jason Dusek
>
>
> ------------------------------
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
>
> End of Beginners Digest, Vol 10, Issue 14
> *****************************************
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/beginners/attachments/20090414/9ace0b1b/attachment-0001.htm
More information about the Beginners
mailing list