Am 01/04/2016 um 11:45 AM schrieb Imants Cekusins:
a newtype for ordered lists
why not: newtype Ordlist a = Ordlist [a]
All nice and dandy, but at first you already need an Ord constraint for your smart constructor -- and a ctor: ordList::(Ord a) => [a]-> OrdList a ordList = OrdList . sort but this is still not the main problem. When you try to define a Functor instance, you'd be tempted to do this (at least I was): instance Functor OrdList where fmap f (OrdList xs) = OrdList $ sort $ map f xs but you can't do this, because of: "No instance for (Ord b) arising from a use of ‘sort’", where b is the return type of f :: (a->b). This does make sense, the function has to return something which can be sorted. So my question is: is it impossible to write a functor instance for ordered lists? It appears so, because a Functor does not impose any constraints of f. But my knowledge is quite limited and maybe a well-set class constraint can fix things.