On 3 Feb 2017, at 21:59, Francesco Ariis <fa-ml@ariis.it> 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?



_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners