[Haskell-cafe] What's the advantage of writing Haskell this way?
John Ky
newhoggy at gmail.com
Tue May 31 09:31:14 CEST 2011
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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