Changes to Typeable

[Edward Kmett]

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 at gmail.com> wrote:

> On 11 February 2012 00:30, John Meacham <john at 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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