I just have passing commentary on your issues. For the bigger questions at the end of the mail I don't have good answers sadly, although just by convenience on question from SO/reddit/web in general I would stay on GHC/base so they can search their questions. On 23/09/17 22:02, erwig wrote:
* Errors resulting from overloading
On GHC 8.2.1 there is a new flag for `:type +d` which uses type-defaulting for giving non-overloaded versions for functions. Prelude> :type +d foldr foldr :: (a -> b -> b) -> b -> [a] -> b Prelude> :type foldr foldr :: Foldable t => (a -> b -> b) -> b -> t a -> b That help to return some of the convenience from before Foldable was in the Prelude. There also -XTypeApplications which lets you play with the signatures Prelude> :set -XTypeApplications Prelude> import Data.Monoid Prelude> import Data.Set Prelude> :type +v foldMap foldMap :: Foldable t => forall m a. Monoid m => (a -> m) -> t a -> m Prelude> :type foldMap @Set foldMap @Set :: Monoid m => (a -> m) -> Set a -> m Prelude> :type foldMap @Set @(Sum _) foldMap @Set @(Sum _) :: Num w => (a -> Sum w) -> Set a -> Sum w Prelude> :type foldMap @Set @(Sum Int) foldMap @Set @(Sum Int) :: (a -> Sum Int) -> Set a -> Sum Int type +v is to see the explicit forall and thus how the TypeApplications get applied. Lastly there is more help to see what instances are available in the form of link in the haddock pages of the classes.
* Errors due to undefined type class instances (specifically, Eq and Show)
I would teach to define instances for those clases because they are easy to do. They work on types of kind * thus are very down to earth. I remember doing this with LYAH. That way they won't see them as magic and actually understand why the compiler is barfing.
* Errors in the context of parametric polymorphism
Those are actually hard. Classes of working on kinds * -> * are what gives haskell the feeling that the class/instances are hard and magical. Playing with them is the only to get comfortable though. -- -- Ruben -- pgp: 4EE9 28F7 932E F4AD