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

31 May 2011

      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  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 
...
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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