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

I think this is because mconcat expects a list. On Tue, May 31, 2011 at 3:31 PM, John Ky 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 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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