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 wrote:

...

From: John Ky 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

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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 ...

Maybe you are looking for a more generic way to concatenate it: There is foldhttp://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Foldab...:: (Foldable t, Monoid m) => t m -> mhttp://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Foldab...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

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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