Re: Magical function to support reflection

The problem is in the reify function: ``` reify :: forall a r. a -> (forall (s :: *). Reifies s a => Proxy s -> r) -> r reify a k = unsafeCoerce (Magic k :: Magic a r) (const a) Proxy ``` here, unsafeCoerce coerces `const a` to type `a`, in the concrete case, to Int. ``` *Main> unsafeCoerce (const 5) :: Int 1099511628032 ``` this is indeed what seems to be the issue: ``` *Main> reify 5 reflect 1099511628032 ``` which is why test1 then shows the wrong result. Also, in the Magic newtype, there’s a `Proxy s`, which afaik doesn’t have the expected runtime representation `a -> r`. (there’s a proxy in the middle, `a -> Proxy -> r`). Changing Magic to ``` newtype Magic a r = Magic (forall (s :: *) . Reifies s a => Tagged s r) ``` now has the correct runtime rep, and the reification can be done by coercing the Magic in to `a -> r`, as such ``` reify' :: a -> (forall (s :: *) . Reifies s a => Tagged s r) -> r reify' a f = unsafeCoerce (Magic f) a ``` the Proxy version is just a convenience, wrapped around the magic one: ``` reify :: forall r a. a -> (forall (s :: *) . Reifies s a => Proxy s -> r) -> r reify a f = reify' a (unproxy f) ``` Here’s the complete file, with the changes that compile and now work: https://gist.github.com/kcsongor/b2f829b2b60022505b7e48b1360d2679 https://gist.github.com/kcsongor/b2f829b2b60022505b7e48b1360d2679 — Csongor

On 20 Jan 2017, at 14:14, Matthew Pickering wrote:

I modified the example on the wiki to compile but I seem to have missed something, could you perhaps point out what I missed?

https://gist.github.com/mpickering/da6d7852af2f6c8f59f80ce726baa864

``` *Main> test1 2 123 441212 441335 ```

On Thu, Jan 19, 2017 at 3:58 AM, David Feuer wrote:

...

I've updated https://ghc.haskell.org/trac/ghc/wiki/MagicalReflectionSupport to reflect both Simon's thoughts on the matter and my own reactions to them. I hope you'll give it a peek.

David Feuer _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

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