Using row kinds in instance declarations

[daniel-chambers]

The PureScript compiler seems to prevent the following monadMine instance from being created:

import Prelude
import Control.Monad.Aff (Aff)
import Network.HTTP.Affjax (AJAX)

class Monad m <= MonadMine m where
  test :: Unit -> m Unit

instance monadMine :: MonadMine (Aff (ajax :: AJAX | e)) where
  test _ = pure unit

(ignore that the ajax effect isn't actually used in the instance, that's just for simplifying the example. Imagine replacing the pure unit call with something that requires that effect)

It gives the following error:

Type class instance head is invalid due to use of type at line 8 col 1
    ( ajax :: AJAX
    | e
    )
  All types appearing in instance declarations must be of the form T a_1 .. a_n, where each type a_i is of the same form.
in type class instance MonadHttp.MonadMine (Aff
                                              ( ajax :: AJAX
                                              | e
                                              )
                                           )

To work around this, one must create a newtype to wrap the type being instanced, and then create the instance for that newtype instead. (Example)

Is there a reason why the PureScript compiler rejects using rows in the instance declaration? It seems like on face value that this should work.

[paf31]

Rows are not allowed in instance heads, although there may be a workaround once we get ~ constraints:

instance monadMine :: (e ~ (ajax :: AJAX)) => MonadMine (Aff e)

There are existing discussions about this in #1510 and #1822.