In practice I've found that working with Tagged is a huge pain relative to working with Proxy.

You usually need to use ScopedTypeVariables or do asTypeOf/asArgOf tricks that are far more complicated than they need to be.

For reference you can compare the internals of reflection before when it used to use Tagged, and after I switched it to use Proxy.

The Proxy version is much simpler.

Tagged works well when you only need one tag and are going to use it for a lot of types. That really isn't the usecase with Typeable most of the time.

-Edward

On Fri, Feb 10, 2012 at 7:35 PM, Bas van Dijk <v.dijk.bas@gmail.com> wrote:
On 11 February 2012 00:30, John Meacham <john@repetae.net> wrote:
> Would it be useful to make 'Proxy' an unboxed type itself? so
>
> Proxy :: forall k . k -> #
>
> This would statically ensure that no one accidentally passes ⊥ as a parameter
> or will get anything other than the unit 'Proxy' when trying to evaluate it.
> So the compiler can unconditionally elide the parameter at runtime. Pretty
> much exactly how State# gets dropped which has almost the same definition.

Or don't use an argument at all:

class Typeable t where
 typeRep :: Tagged t TypeRep

newtype Tagged s b = Tagged { unTagged :: b }

See:

http://hackage.haskell.org/packages/archive/tagged/0.2.3.1/doc/html/Data-Tagged.html

Bas

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