I'm playing around with backpack, trying to rewrite existing libraries. My end
goal is to see if Backpack could improve the primitive library. Right now, the
primitive library (in my opinion) relies too heavily on specialization of its
type classes, so I think Backpack could help. However, I seem to be running
into a limitation. I am wondering if it is a fundamental limitation, if perhaps
there is a workaround, or if Backpack could be improved to support this use-case.

Instead of primitive, I will take the simpler example: semigroup, which also
shows this limitation. Let's convert the Semigroup class to a backpack signature:

     unit indef where
       signature Semigroup where
         import Prelude hiding ((<>))
         data T
         (<>) :: T -> T -> T

The problem is how to implement this signature with the type of polymorphic
lists. It is easy to implement it for concrete lists like strings:

     unit string where
      module Semigroup where
       import Prelude hiding ((<>))
       type T = String
       (<>) :: T -> T -> T
       (<>) = (++)

It is also possible to implement it in terms of another signature:

     unit list where
       signature Elem where
         data A

       module Semigroup where
         import Prelude hiding ((<>))
         import Elem
         type T = [A]
         (<>) :: T -> T -> T
         (<>) = (++)

This is still problematic, because it is annoying that this new type A needs to
be instantiated each time you want to use it. However, even more importantly,
if I want to translate the 'PrimMonad' class to a Backpack signature then the
'ST s' instance needs a polymorphic type variable 's', which cannot be made
concrete.

And do note that I want the monad to be concrete for performance reasons, but
the 's' parameter doesn't have to be concrete, because it is a phantom
parameter anyway. And for lists making the 'a' parameter concrete also would
not improve performance as far as I know.

One possible way to fix this is to add a type variable in the 'Semigroup'
signature, but then I think it becomes impossible to write the 'String'
instance and sometimes you need more than one new type variable such as with
the 'ReaderT r (ST s)' instance of 'PrimMonad'.

In OCaml you can still kind of work around this problem by creating local
instances inside functions. That trick still allows you to write a polymorphic
concatenation function using a monoid signature (taken from [1]):

     let concat (type a) xs =
       let module MU = MonoidUtils (ListMonoid(struct type t = a end)) in
       MU.concat xs;;

So, I'm wondering if it would be possible to "generalise" over indefinite
Backpack types such as 'A' in the 'Elem' signature above or if we can at least
implement something which enables the same trick that you can use in OCaml.

Thanks,

Jaro

[1] https://blog.shaynefletcher.org/2017/05/more-type-classes-in-ocaml.html
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