Data.Dynamic: Any vs existential

[Dan Doel]

On Mon, Mar 10, 2014 at 6:35 AM, Roman Cheplyaka <roma at ro-che.info> wrote:

> Ok, one reason is that the TypeRep won't be cached in the Dynamic value.
> Even in GHC 7.8 Typeable is defined as
>
>   class Typeable a where
>     typeRep# :: Proxy# a -> TypeRep
>
> instead of
>
>   class Typeable a where
>     typeRep :: Tagged a TypeRep
>
> Why? Is this an oversight?
>

I talked with Ed about this, and he noted that this might be a false
problem. Certainly, the Proxy# is never actually used, so as long as the
generated instances are of the form:

    typeRep# = let tr = ... in \_ -> tr

the TypeReps are shared, and are at most a bit of indirection away. Also,
how different are (Tagged tr :: Tagged a TypeRep) and ((\_ -> tr) :: Proxy#
a -> TypeRep) at a low level? I know the constructor of the former
disappears in core, but don't know what specifically happens with 0-width
fields like Proxy#. Is the latter slightly less efficient? If so, can the
optimizer eliminate the difference for cases like this?

-- Dan
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