[Haskell-cafe] What's the advantage of writing Haskell this way?

[John Ky]

Hi Brandon,

Thanks for your suggestion.  I'm a little stuck as adding Monad and
MonadPlus in my instance declaration doesn't seem sufficient.  I know
mconcat comes from Monoid, but I don't know how to put that in.

data Stream m a
= Chunks (m a)
| EOF
deriving (Show, Eq)

instance (Monad m, MonadPlus m) => Monoid (Stream m a) where
mempty = Chunks mempty
mappend (Chunks xs) (Chunks ys) = Chunks (xs `mappend` ys)
mappend _ _ = EOF

instance (Monad m, MonadPlus m) => Monad (Stream m) where
return = Chunks . return
Chunks xs >>= f = mconcat (fmap f xs)
EOF >>= _ = EOF

Iteratee.hs:28:25:
    No instance for (Monoid (m a))
      arising from a use of `mempty'
    Possible fix: add an instance declaration for (Monoid (m a))
    In the first argument of `Chunks', namely `mempty'
    In the expression: Chunks mempty
    In an equation for `mempty': mempty = Chunks mempty

Iteratee.hs:29:54:
    No instance for (Monoid (m a))
      arising from a use of `mappend'
    Possible fix: add an instance declaration for (Monoid (m a))
    In the first argument of `Chunks', namely `(xs `mappend` ys)'
    In the expression: Chunks (xs `mappend` ys)
    In an equation for `mappend':
        mappend (Chunks xs) (Chunks ys) = Chunks (xs `mappend` ys)

Iteratee.hs:34:43:
    Could not deduce (m ~ [])
    from the context (Monad m, MonadPlus m)
      bound by the instance declaration at Iteratee.hs:32:10-51
      `m' is a rigid type variable bound by
          the instance declaration at Iteratee.hs:32:17
    Expected type: [a]
      Actual type: m a
    In the second argument of `fmap', namely `xs'
    In the first argument of `mconcat', namely `(fmap f xs)'
    In the expression: mconcat (fmap f xs)
Failed, modules loaded: none.

Cheers,

-John

On 31 May 2011 00:38, Brandon Moore <brandon_m_moore at yahoo.com> wrote:

> >From: John Ky <newhoggy at gmail.com>
> >Sent: Monday, May 30, 2011 8:01 AM
> >
> >Hi all,
> >
> >I'm trying to learn about enumerators by reading this paper and came
> across some code on page 2 that I found hard to digest, but I think I
> finally got it:
>
>
> Hi John. These programs should behave identically, and I think your version
> should be preferred.
> This first code uses some class methods like mconcat, but it seems to
> always be used on
> the list in Chunks, so it will only ever use the definition for list, which
> is equivalent to what
> you wrote directly in the second code.
>
> The result may not be useful, but to understand this more thoroughly you
> might
> try parametrizating the definition of Stream so the use of more general
> operators
> actually means something. Perhaps
>
> data Stream m a =
>   Chunks (m a)
>   | EOF
>
> I think you would want Monad and MonadPlus on m.
>
>
> >import Data.Monoid
> >>
> >>
> >>data Stream a
> >>= Chunks [a]
> >>| EOF
> >>deriving (Show, Eq)
> >>
> >>
> >>instance Monad Stream where
> >>return = Chunks . return
> >>Chunks xs >>= f = mconcat (fmap f xs)
> >>EOF >>= _ = EOF
> >>
> >>
> >>instance Monoid (Stream a) where
> >>mempty = Chunks mempty
> >>mappend (Chunks xs) (Chunks ys) = Chunks (xs ++ ys)
> >>mappend _ _ = EOF
> >
> >
> >I guess, it shows my lack of experience in Haskell, but my question is,
> why is writing the code this way preferred over say writing it like this:
> >
> >
> >import Data.Monoid
> >>
> >>
> >>data Stream a
> >>= Chunks [a]
> >>| EOF
> >>deriving (Show, Eq)
> >>
> >>
> >>instance Monad Stream where
> >>return x = Chunks [x]
> >>Chunks xs >>= f = mconcat (fmap f xs)
> >>EOF >>= _ = EOF
> >>
> >>
> >>instance Monoid (Stream a) where
> >>mempty = Chunks []
> >>mappend (Chunks xs) (Chunks ys) = Chunks (xs ++ ys)
> >>mappend _ _ = EOF
> >
> >
> >Cheers,
> >
> >
> >-John
> >
> >
> >_______________________________________________
> >Haskell-Cafe mailing list
> >Haskell-Cafe at haskell.org
> >http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
> >
> >
>
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