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

[Lyndon Maydwell]

Heh. Looks like there will be about five class constraints, but it
will still be more general.

There must be some higher level abstraction that is less ugly.

On Tue, May 31, 2011 at 3:45 PM, Yves Parès <limestrael at gmail.com> wrote:
> Maybe you are looking for a more generic way to concatenate it:
> There is fold :: (Foldable t, Monoid m) => t m -> m 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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