Ryan Scott
Hi Chris,
The change to ($)'s type is indeed intentional. The short answer is that ($)'s type prior to GHC 8.0 was lying a little bit. If you defined something like this:
unwrapInt :: Int -> Int# unwrapInt (I# i) = i
... Hello everyone, While this thread continues to smolder, it seems that the arguments relevant to the levity polymorphism change have been sussed out. Now seems like a good time to review what we have all learned, * In 7.10 and earlier the type of ($) is a bit of a lie as it did not reflect the fact that the result type was open-kinded. ($) also has magic to allow impredicative uses, although this is orthogonal to the present levity discussion. * the type of ($) has changed to become more truthful in 8.0: we now capture lifted-ness in the type system with the notion of Levity. * there is widespread belief that the new type is too noisy and obfuscates the rather simple concept embodied by ($). This is especially concerning for those teaching and learning the language. * One approach to fix this would be to specialize ($) for lifted types and introduce a new levity polymorphic variant. This carries the potential to break existing users of ($), although it's unclear how much code this would affect in practice. * Another approach would be to preserve the current lie with pretty-printer behavior. This would be relatively easy to do and would allow us to avoid breaking existing users of ($). This, however, comes at the expense of some potential confusion when polymorphism is needed. * There are further questions regarding the appropriate kinds of (->) and (.) [1] * Incidentally, there is a GHC or Haddock bug [2] which causes kind signatures to be unnecessarily shown in documentation for some types, exposing levities to the user. The current plan to address this situation is as follows, * Introduce [3] a flag, -fshow-runtime-rep, which when disabled will cause the pretty-printer to instantiate levity-polymorphic types as lifted (e.g. resulting in *). This flag will be off by default, meaning that users will in most cases see the usual lifted types unless they explicitly request otherwise. * Fix the GHC/Haddock bug, restoring elision of unnecessary kind signatures in documentation. * In the future we should seriously consider introducing an alternate Prelude for beginners As far as I can tell from the discussion, this was an acceptable solution to all involved. If there are any remaining objections or concerns let's discuss them in another thread. Thanks to everyone who contributed to this effort. Cheers, - Ben [1] https://ghc.haskell.org/trac/ghc/ticket/10343#comment:27 [2] https://ghc.haskell.org/trac/ghc/ticket/11567 [3] https://ghc.haskell.org/trac/ghc/ticket/11549