Yes, that’s exactly what I meant to write!!! :) Thank you Francesco. Mike

On 3 Feb 2017, at 21:59, Francesco Ariis wrote:

On Fri, Feb 03, 2017 at 09:47:04PM +0000, mike h wrote:

...

I have

---------- import qualified Data.Map as M

type Link a = (a, Int) data MChain a = Map a [Link a] deriving (Show)

-------------------

and want to make a Monoid of MChain. So I have

------------------- instance Monoid (MChain a) where mempty = M.empty mappend = undefined -------------------

this won’t compile and I need M.empty to be Map a [Link a]

Hello Mike, I think the error lies in the confusion between `type` and `data` declaration.

type Something = Int

but

data Something = SomeConstructor Int

So I bet you wanted to write

data MChain a = MChain (M.Map a [Link a]) deriving (Show)

`M.empty` returns a Map.

λ> :t M.empty M.empty :: M.Map k aj

Hence this will work:

instance Monoid (MChain a) where mempty = MChain M.empty mappend = undefined

Does this help?

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