I was just writing a blog post related to that.

You’d need to use

newtype GTag xs a = GTag { unTag :: NS ((:~:) a) xs }

Then you can recover needed type equality.

https://gist.github.com/phadej/bfdffc3bd5ba6ce4f586bb988a8d399c

Alternatively:

You could use `NS Proxy xs`, but than it’s much easier to make a mistake.

- Oleg

On 20 Nov 2016, at 19:27, Anders Papitto <anderspapitto@gmail.com> wrote:

Hello! I'm having a lot of trouble writing a Data.GADT.Compare.GEq instance - can anyone help me fill in the blank? For context - I'm generating Tag types automatically for using with DSum, and I need a GEq instance. That's part of an attempt to add efficient rendering of sum types to Reflex/Reflex-Dom (https://github.com/anderspapitto/reflex-sumtype-render/blob/master/src/ReflexHelpers.hs).

I put this question on stackoverflow as well a day ago (http://stackoverflow.com/questions/40698207/how-can-i-write-this-geq-instance).

Here's the code (it's a full, standalone file - you can copy it into Foo.hs and run ghc to see the full error I'm facing). The error I get is that when I try to recursively call geq on the unwrapped x and y, I can't because ghc considers them to have different types - Quux a and Quux b. However, the whole point of why I'm trying to call geq is to see if a and b are the same, so I'm pretty confused.

Note that I'm making use of the generics-sop library, which is where NP and NS and I come from.

{-# LANGUAGE GADTs #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE RankNTypes #-}

module Foo where

import Data.GADT.Compare
import Generics.SOP
import qualified GHC.Generics as GHC

data Quux i xs where Quux :: Quux (NP I xs) xs

newtype GTag t i = GTag { unTag :: NS (Quux i) (Code t) }

instance GEq (GTag t) where
  -- I don't know how to do this
  geq (GTag (S x)) (GTag (S y)) =
    let _ = x `geq` y
    in undefined

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