#14917: Allow levity polymorphism in binding position -------------------------------------+------------------------------------- Reporter: andrewthad | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: | LevityPolymorphism Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by andrewthad): There was a reddit discussion[1] that reminded me of this, and it got a tangential but related idea going in my mind. I wonder if there's any connection between user-defined functions with a compulsory unfolding and `UNPACK` on polymorphic fields in a data constructor (currently disallowed because they result in code that cannot be compiled). Roughly, the idea was that a compulsory unfolding would be required for functions that target data types with unpacked polymorphic fields. For example: {{{ data Foo a = Foo {-# UNPACK -#} !Int {-# UNPACK #-} !a {-# INLINE MANDATE useFoo #-} useFoo :: Foo a -> a useFoo (Foo _ a) = a }}} This would also lift the restriction on levity-polymorphic fields in a data constructor. There are a bunch of other problems with this that may not be possible to resolve. How does pattern matching actually work with this? How can `Foo Word` be stored inside of another data type? Anyway, I'm not convinced this could actually go anywhere, but I thought I'd jot it down. [1] https://www.reddit.com/r/haskell/comments/8a5w1n/new_package_unpackedcontain... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14917#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler