[GHC] #14917: Allow levity polymorphism in binding position

[GHC]

#14917: Allow levity polymorphism in binding position
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        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:                    |
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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_unpackedcontainers/

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14917#comment:14>
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