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?