Magical function to support reflection

Simon Peyton Jones simonpj at
Tue Jan 17 21:41:09 UTC 2017

To make the reflection library work in a fully type-safe manner would take 1-3 additional wired ins that would consist of well-typed core. The stuff David is proposing above would be more general but less safe.
The approach I proposed below looks general, safe, and performant.  Or not?

To make progress it’d be good to update the wiki page, both in the light of the recent discussion, and with pointers to related packages, motivation, papers, to set the context


From: Edward Kmett [mailto:ekmett at]
Sent: 17 January 2017 19:43
To: Simon Peyton Jones <simonpj at>
Cc: David Feuer <david.feuer at>; ghc-devs <ghc-devs at>
Subject: Re: Magical function to support reflection

That is the paper the reflection library API is based on.

However, doing it the way mentioned in that paper (after modifying it to work around changes with the inliner for modern GHC) is about 3 orders of magnitude slower. We keep it around in reflection as the 'slow' path for portability to non-GHC compilers, and because that variant can make a form of Typeable reflection which is needed for some Exception gimmicks folks use.

The current approach, and the sort of variant that David is pushing above, is basically free, as it costs a single unsafeCoerce. To make the reflection library work in a fully type-safe manner would take 1-3 additional wired ins that would consist of well-typed core. The stuff David is proposing above would be more general but less safe.


On Tue, Jan 17, 2017 at 10:45 AM, Simon Peyton Jones <simonpj at<mailto:simonpj at>> wrote:
David says that this paper is relevant<>


From: David Feuer [mailto:david.feuer at<mailto:david.feuer at>]
Sent: 14 January 2017 00:50
To: Simon Peyton Jones <simonpj at<mailto:simonpj at>>
Cc: ghc-devs <ghc-devs at<mailto:ghc-devs at>>; Edward Kmett <ekmett at<mailto:ekmett at>>
Subject: RE: Magical function to support reflection

I need to look through a bit more of this, but explicit type application certainly can be avoided using Tagged. Once we get the necessary magic, libraries will be able to come up with whatever interfaces they like. My main concern about the generality of

reify# :: forall r. (RC a => r) -> a -> r

(as with the primop type Edward came up with) is that it lacks the `forall s` safety mechanism of the reflection library. Along with its key role in ensuring class coherence[*], that mechanism also makes it clear what specialization is and is not allowed to do with reified values. Again, I'm not sure it can mess up the simpler/more general form you and Edward propose, but it makes me nervous.

[*] Coherence: as long as an instance of Reifies S A exists for some concrete S::K, users can't incoherently write a polymorphic Reifies instance for s::K.

On Jan 13, 2017 7:33 PM, "Simon Peyton Jones" <simonpj at<mailto:simonpj at>> wrote:
David, Edward
Here’s my take on this thread about reflection.   I’ll ignore Tagged and the ‘s’ parameter, and the proxy arguments, since they are incidental.
I can finally see a reasonable path; I think there’s a potential GHC proposal here.

First thing: PLEASE let's give a Core rendering of whatever is proposed. If it's expressible in Core that's reassuring.  If it requires an extension to Core, that's a whole different thing.

Second.  For any particular class, I think it's easy to express reify in Core.  Example (in Core):

reifyTypeable :: (Typeable a => b) -> TypeRep a -> b

reifyTypable k = k |> co
where co is a coercion that witnesses

  co :: (forall a b. Typeable a => b) ~ forall a b. (TypeRep a -> b)

Third.  This does not depend, and should not depend, on the fact that single-method classes are represented with a newtype.  E.g. if we changed Typeable to be represented with a data type thus (in Core)

data Typeable a = MkTypeable (TypeRep a)
using data rather than newtype, then we could still write reifyTypable.

reifyTypeable :: (Typeable a => b) -> TypeRep a -> b

reifyTypable = /\ab. \(f :: Typeable a => b). \(r :: TypeRep a).

               f (MkTypeable r)
The efficiency of newtype is nice, but it’s not essential.

Fourth.   As you point out, reify# is far too polymorphic. Clearly you need reify# to be a class method!  Something like this

class Reifiable a where

  type RC a :: Constraint  -- Short for Reified Constraint

  reify# :: forall r. (RC a => r) -> a -> r
Now (in Core at least) we can make instances

instance Reifiable (TypeRep a) where

  type RC (TypeRep a) = Typeable a

  reify# k = k |> co  -- For a suitable co
Now, we can’t write those instances in Haskell, but we could make the ‘deriving’ mechanism deal with it, thus:

deriving instance Reifiable (Typeable a)
You can supply a ‘where’ part if you like, but if you don’t GHC will fill in the implementation for you.  It’ll check that Typeable is a single-method class; produce a suitable implementation (in Core, as above) for reify#, and a suitable instance for RC. Pretty simple.   Now the solver can use those instances.
There are lots of details

•        I’ve used a single parameter class and a type function, because the call site of reify# will provide no information about the ‘c’ in (c => r) argument.

•        What if some other class has the same method type?  E.g. if someone wrote

class MyTR a where op :: TypeRep a

would that mess up the use of reify# for Typeable?   Well it would if they also did

deriving instance Reifiable (MyTR a)

And there really is an ambiguity: what should (reify# k (tr :: TypeRep Int)) do?  Apply k to a TypeRep or to a MyTR?  So a complaint here would be entirely legitimate.

•        I suppose that another formulation might be to abstract over the constraint, rather than the method type, and use explicit type application at calls of reify#.  So

class Reifiable c where

  type RL c :: *

  reify# :: (c => r) -> RL c -> r

Now all calls of reify# would have to look like

reify# @(Typeable Int) k tr

Maybe that’s acceptable.  But it doesn’t seem as nice to me.

•        One could use functional dependencies and a 2-parameter type class, but I don’t think it would change anything much.  If type functions work, they are more robust than fundeps.

•        One could abstract over the type constructor rather than the type.  I see no advantage and some disadvantages

class Reifiable t where

  type RC t :: * -> Constraint  -- Short for Reified Constraint

  reify# :: forall r. (RC t a => r) -> t a -> r

|  -----Original Message-----

|  From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of David

|  Feuer

|  Sent: 11 December 2016 05:01

|  To: ghc-devs <ghc-devs at<mailto:ghc-devs at>>; Edward Kmett <ekmett at<mailto:ekmett at>>

|  Subject: Magical function to support reflection


|  The following proposal (with fancier formatting and some improved

|  wording) can be viewed at



|  Using the Data.Reflection has some runtime costs. Notably, there can be no

|  inlining or unboxing of reified values. I think it would be nice to add a

|  GHC special to support it. I'll get right to the point of what I want, and

|  then give a bit of background about why.


|  === What I want


|  I propose the following absurdly over-general lie:


|  reify# :: (forall s . c s a => t s r) -> a -> r


|  `c` is assumed to be a single-method class with no superclasses whose

|  dictionary representation is exactly the same as the representation of `a`,

|  and `t s r` is assumed to be a newtype wrapper around `r`. In desugaring,

|  reify# f would be compiled to f at S, where S is a fresh type. I believe it's

|  necessary to use a fresh type to prevent specialization from mixing up

|  different reified values.


|  === Background


|  Let me set up a few pieces. These pieces are slightly modified from what the

|  package actually does to make things cleaner under the hood, but the

|  differences are fairly shallow.


|  newtype Tagged s a = Tagged { unTagged :: a }


|  unproxy :: (Proxy s -> a) -> Tagged s a

|  unproxy f = Tagged (f Proxy)


|  class Reifies s a | s -> a where

|    reflect' :: Tagged s a


|  -- For convenience

|  reflect :: forall s a proxy . Reifies s a => proxy s -> a reflect _ =

|  unTagged (reflect' :: Tagged s a)


|  -- The key function--see below regarding implementation reify' :: (forall s

|  . Reifies s a => Tagged s r) -> a -> r


|  -- For convenience

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

|  reify' (unproxy f) a


|  The key idea of reify' is that something of type


|  forall s . Reifies s a => Tagged s r


|  is represented in memory exactly the same as a function of type


|  a -> r


|  So we can currently use unsafeCoerce to interpret one as the other.

|  Following the general approach of the library, we can do this as such:


|  newtype Magic a r = Magic (forall s . Reifies s a => Tagged s r) reify' ::

|  (forall s . Reifies s a => Tagged s r) -> a -> r reify' f = unsafeCoerce

|  (Magic f)


|  This certainly works. The trouble is that any knowledge about what is

|  reflected is totally lost. For instance, if I write


|  reify 12 $ \p -> reflect p + 3


|  then GHC will not see, at compile time, that the result is 15. If I write


|  reify (+1) $ \p -> reflect p x


|  then GHC will never inline the application of (+1). Etc.


|  I'd like to replace reify' with reify# to avoid this problem.


|  Thanks,

|  David Feuer

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