[Haskell-cafe] EduHaskell

[Ruben Astudillo]

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