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

[Yves Parès]

Maybe you are looking for a more generic way to concatenate it:
There is fold<http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Foldable.html#v:fold>::
(Foldable t, Monoid m) => t
m -> m<http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Foldable.html#v:fold>in
Data.Foldable, but it would add another Foldable constraint.

You search a function like:
concatMPlus :: (MonadPlus m, Monoid a) => m a -> a
but this cannot exist ;) ("m a -> m a" would, but not "m a -> a")


2011/5/31 Lyndon Maydwell <maydwell at gmail.com>

> I think this is because mconcat expects a list.
>
> On Tue, May 31, 2011 at 3:31 PM, John Ky <newhoggy at gmail.com> wrote:
> > Thanks Malcom.
> > I suspected that much, so I added it:
> > data Stream m a
> > = Chunks (m a)
> > | EOF
> > deriving (Show, Eq)
> > instance (Monad m, MonadPlus m, Monoid (m a)) => 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
> > This gives me the error:
> > Iteratee.hs:30:10:
> >     Non type-variable argument in the constraint: Monoid (m a)
> >     (Use -XFlexibleContexts to permit this)
> >     In the context: (Monad m, MonadPlus m, Monoid (m a))
> >     While checking the context of an instance declaration
> >     In the instance declaration for `Monoid (Stream m a)'
> > So I run with the new flag:
> > ghci -XFlexibleContexts Iteratee.hs
> > Then I get the following error instead:
> > Iteratee.hs:37:43:
> >     Could not deduce (m ~ [])
> >     from the context (Monad m, MonadPlus m)
> >       bound by the instance declaration at Iteratee.hs:35:10-51
> >       `m' is a rigid type variable bound by
> >           the instance declaration at Iteratee.hs:35:17
> >     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)
> > Which is complaining about the line I highlighted above.  So I try:
> > data Stream m a
> > = Chunks (m a)
> > | EOF
> > deriving (Show, Eq)
> > instance (Monad m, MonadPlus m, Monoid (m a)) => Monoid (Stream m a)
> where
> > mempty = Chunks mempty
> > mappend (Chunks xs) (Chunks ys) = Chunks (xs `mappend` ys)
> > mappend _ _ = EOF
> > instance (Monad m, MonadPlus m, Monoid (m a)) => Monad (Stream m) where
> > return = Chunks . return
> > Chunks xs >>= f = mconcat (fmap f xs)
> > EOF >>= _ = EOF
> > But the same trick doesn't work:
> > Iteratee.hs:35:10:
> >     Variable occurs more often in a constraint than in the instance head
> >       in the constraint: Monoid (m a)
> >     (Use -XUndecidableInstances to permit this)
> >     In the instance declaration for `Monad (Stream m)'
> > Is that because I don't use a on the right hand side of =>?
> > Cheers,
> > -John
> > On 31 May 2011 15:54, Malcolm Wallace <malcolm.wallace at me.com> wrote:
> >>
> >> instance (Monad m, MonadPlus m) => Monoid (Stream m a) where
> >>
> >> mempty = Chunks mempty
> >> mappend (Chunks xs) (Chunks ys) = Chunks (xs `mappend` ys)
> >> mappend _ _ = EOF
> >>
> >> Iteratee.hs:28:25:
> >>     No instance for (Monoid (m a))
> >>       arising from a use of `mempty'
> >>
> >> There is a clue in the first part of the error message.  Add the
> required
> >> instance as part of the predicate:
> >> instance (Monad m, MonadPlus m, Monoid (m a)) => Monoid (Stream m a)
> where
> >> ...
> >
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