[GHC] #12989: ($) can have a more general type

[GHC]

#12989: ($) can have a more general type
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        Reporter:  dfeuer            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:                    |  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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Changes (by simonpj):

 * keywords:   => LevityPolymorphism


Comment:

 Worth reading our [https://www.microsoft.com/en-us/research/publication
 /levity-polymorphism/ levity polymorphism paper].

 `good` is OK because no value of type `a` is manipulated by the code for
 `good` (see the paper).  NB: care is needed during optimisation: eta
 expansion (to get `bad`) is not allowed.

 So, yes, perhaps `($)` could be defied like `good`, as an arity-1
 function.  Then it could have the more generous type.

 I'll add this to the levity-polymorphism pile; see
 [wiki:LevityPolymorphism]

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12989#comment:3>
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