Re: Changes to Typeable

Edward, it was my impression that you have to use ScopedTypeVariables or other tricks to work with Proxy to. For example say I want to write: typeOf :: Typeable a => a -> TypeRep With Proxy I can either write: {-# LANGUAGE ScopedTypeVariables #-} typeOf :: forall a. Typeable a => a -> TypeRep typeOf _ = typeRep (Proxy :: Proxy a) or without extensions: typeOf :: Typeable a => a -> TypeRep typeOf x = typeRep (p x) where p :: b -> Proxy b p _ = Proxy But with Tagged the situation is similar: {-# LANGUAGE ScopedTypeVariables #-} typeOf :: forall a. Typeable a => a -> TypeRep typeOf _ = unTagged (typeRep :: Tagged a TypeRep) or without extensions: typeOf :: Typeable a => a -> TypeRep typeOf x = unTagged (t x) where t :: Typeable b => b -> Tagged b TypeRep t _ = typeRep Where is the "huge pain" you are talking about? I do have to admit that the Proxy versions are slightly smaller and easier to read. Bas On 12 February 2012 00:56, Edward Kmett wrote:

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 wrote:

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On 11 February 2012 00:30, John Meacham 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-Tag...

Bas

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