[GHC] #1965: Allow unconstrained existential contexts in newtypes

[GHC]

#1965: Allow unconstrained existential contexts in newtypes
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        Reporter:  guest             |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  6.8.1
      Resolution:                    |             Keywords:
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 dfeuer):

 Replying to [comment:9 simonpj]:
 > Anything involving existentials is going to be hard to implement using
 newtype directly.  But as 'mokus' says, it might be possible to make a
 guarantee, right in the code generator, that certain sorts of data types
 do not allocate a box.  The conditions are, I think, very nearly as
 'mokus' says:
 >  1. Only one constructor
 >  2. Only one field with nonzero width in that constructor (counting
 constraints as fields)
 >  3. That field is marked strict
 >  4. That field has a boxed (or polymorphic) type
 > I think this'd be do-able.  The question is how important it is in
 practice; it's one more thing to maintain.

 I would like to have something like this very much! Among other things,
 it's one possible way to make `IntMap` nicer. One potential extension: I
 think constraints only need to count as fields if any of them are classes
 that have methods, or whose superclasses have methods. In particular, it
 could be very useful to have equality constraints involving type families.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/1965#comment:28>
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