#9429: Alternative to type family Any -------------------------------------+------------------------------------- Reporter: mboes | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.9 Component: Compiler | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: 9097, 9380 None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by mboes): Replying to [comment:10 carter]:
ok, now if we had a hypothetical `PolyTypeable` would you have other remaining needs for Any?
It occurs to me as I'm saying this, that to some extent PolyTypeable is
I myself wouldn't. I don't know of other use cases beyond the ones internal to GHC. possible today via data/newtype wrapping a polymorphic type! For you use case, Is that a viable solution today (aesthetics aside) or is there a fundamental barrier to that being suitable? Interesting idea. But I'm afraid it isn't a solution without its problems, beyond the mere verbosity that it might induce. If I understand your idea correctly, you're saying that if you want to send say `bind`, then you introduce the following type and wrap `bind`: {{{ data Dict c = c => Dict newtype BindWrap = BindWrap (forall a m. Dict (Monad m) -> m a -> (a -> m b) -> m b) bindWrap = BindWrap $ \Dict -> (>>=) }}} Now, you can send a label for `bindWrap` to the remote side, along with the `TypeRep` for `BindWrap`. But you can't compose `bindWrap`. That is, `bindWrap` is a "static value" in the sense that it doesn't depend on any other runtime "dynamic" value. In other words, it has no free variables. If we say that labels for static values `x :: a` have type `Static a`, then I can't combine `bindWrap` with say `action :: Static [Int]` using {{{ staticApply :: Static (a -> b) -> Static a -> Static b }}} because `bindWrap` is of atomic type. The only reasonable type that I can give to any label for it is `Static BindWrap`. But that's not a `Static` with an arrow type index. So while you can send `bindWrap`, you can't send compositions of it with other `Static` things. This is an important feature for many Cloud Haskell use cases. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9429#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler