Re: [Haskell-cafe] difficulty writing GEq instance

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 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 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/Refle... https://github.com/anderspapitto/reflex-sumtype-render/blob/master/src/Refle...).

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

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 _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.