Typeable Reflection

Question

Typeable Reflection

Closed this issue 9 years ago · 9 comments

It would be extremely useful to be able to get a typeRep for reflected types. Specifically, I'm hoping that (Reifies s a, Typeable a) => Typeable s, where show $ typeRep (Proxy :: Proxy s) outputs a unique ID, perhaps using TemplateHaskell support for global names, or something else like Data.Unique.ID.

I don't know much about the magic going on in reify, but is something like this doable?

Answer 1 · 2014-05-24T03:34:36.000Z

i'm not sure this is possible with the current main operation, reify

Consider the type

reify :: forall a r. a -> (forall s. Reifies s a => Proxy s -> r) -> r

in this context, s is made unique via a existential, and is some sense both unique and local.

could you explain your use case?

Answer 2 · 2014-05-24T03:44:26.000Z

As you might expect, it's for dynamic typing. Specifically, I'm trying to store a type in an environment with the syntactic library. Unfortunately, I don't know much about how this would work as the Typeable code hasn't been implemented yet.

I do know it would require a Typeable instance for storing our data type newtype Zq q = Zq Int, which represents modular arithmetic. We're using a reified modulus, so a Typeable instance for Zq q would require Typeable q. You can see a bit more detail at my question here, though that refers to GHC 7.6, and I'm using 7.8 now.

Answer 3 · 2014-05-24T04:04:29.000Z

try out just using autoderivetypeable in 7.8

per se everything is typeable in ghc 7.8... and it might change to being "autotypedable by default" for 7.10

Answer 4 · 2014-05-24T04:40:12.000Z

I'm not sure what you mean.

{-# LANGUAGE AutoDeriveTypeable #-}

import Data.Reflection
import Data.Typeable

main :: IO ()
main = reify (3 :: Int) (\ p -> print $ show $ typeRep p)

does not compile in GHC 7.8, it says:

Could not deduce (Typeable s) arising from a use of ‘typeRep’
from the context (Reifies s Int)

Similarly, defining the data type Zq, I can print typeRep (Proxy :: Proxy Zq), but not typeRep (Proxy :: Proxy (Zq q)) for a reified q.

Answer 5 · 2014-05-24T22:10:54.000Z

lens currently includes a form of Typeable Reflection.

reifyTypeable :: Typeable a => a -> (forall s. (Typeable s, Reifies s a) => Proxy s -> r) -> r

http://hackage.haskell.org/package/lens-4.1.2/docs/Control-Lens-Internal-Reflection.html

I could in theory backport it here to the main reflection project.

Answer 6 · 2014-05-24T22:12:21.000Z

Unfortunately, it is based on something close to the original approach to reflection by Oleg Kiselyov and Chung-chieh Shan. That means it is a couple of orders of magnitude slower than normal reflection.

Answer 7 · 2015-07-08T21:09:34.000Z

A form of typeable reflection is implemented inside of lens these days. It was needed to make Control.Exception.Lens work after changes were made to Typeable.

http://hackage.haskell.org/package/lens-4.11.1/docs/Control-Lens-Internal-Reflection.html

We could think about moving it back home to here.

Answer 8 · 2016-01-12T22:51:44.000Z

It's been moved, so this issue can, technically, be closed. However, the code has no comments in it and is far from obvious. The Typeable a constraint seems very strange from the outside; everything else seems strange from the inside.

Answer 9 · 2016-01-13T02:19:35.000Z

The implementation does what it needs to do to let me fabricate Exception instances in a post-AutoDeriveTypeable world. I could definitely stand to document it better, but it is basically the old slow path with a slight tweak.