Perhaps this is not what you had in mind, but we could write a signature like
{-# LANGUAGE KindSignatures #-}
signature Mystery where
import Data.Kind
import Control.Monad
data MysteryMonad :: Type -> Type
instance Functor MysteryMonad
instance Applicative MysteryMonad
instance Monad MysteryMonad
instance PrimMonad MysteryMonad
that reused the existing PrimMonad class. Code could depend on that signature without being tied to a concrete monad (this would make the package that has the code "indefinite"). Once we compiled the indefinite code against an actual implementation, it would be optimized as if we had used concrete types from the beginning.
One problem with this solution is that it leaves out ST. If we wanted to make it work with ST, one possible hack would be to define the signature like this
data MysteryMonad :: Type -> Type -> Type
instance Functor (MysteryMonad s)
instance Applicative (MysteryMonad s)
instance Monad (MysteryMonad s)
instance PrimMonad (MysteryMonad s)
And then use some kind of newtype adapter with a phantom type for non-ST monads:
module Mystery where
type MysteryMonad = W IO
newtype W m a b = W (m b) deriving newtype (Functor, Applicative, Monad)
But perhaps it would complicate things too much.
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Today's Topics:
1. Backpack: polymorphic instantation? (Jaro Reinders)
----------------------------------------------------------------------
Message: 1
Date: Sat, 11 Sep 2021 11:58:26 +0200
From: Jaro Reinders <jaro.reinders@gmail.com>
To: Haskell Cafe <haskell-cafe@haskell.org>
Subject: [Haskell-cafe] Backpack: polymorphic instantation?
Message-ID: <3fb19986-f998-61c7-8c56-482b1647e2eb@gmail.com>
Content-Type: text/plain; charset=utf-8; format=flowed
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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