[GHC] #11383: CAFs lose sharing due to implicit call stacks

[GHC]

#11383: CAFs lose sharing due to implicit call stacks
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        Reporter:  simonmar          |                Owner:  gridaphobe
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  8.0.1-rc1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11298            |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by gridaphobe):

 * related:   => #11298


Comment:

 As you said, this is really just the monomorphism restriction (or in this
 case the lack thereof) at work. The two pieces that contribute to this
 behavior are:

 1. `error` is now overloaded with an implicit parameter, the call-stack.
 2. GHC infers implicit parameters (including call-stacks) as needed,
 unless there's an explicit signature or the monomorphism restriction
 applies.

 There's a similar report in #11298, see the two definitions of
 `fooHelper`.

 We could avoid this (potentially confusing) behavior for the most part by
 not inferring call-stacks for top-level definitions. I'm a bit reluctant
 to do so because it makes call-stacks less like implicit parameters, and
 would require another special case in the type-checker. One of the things
 I like about the current version of the call-stack solver is how similar
 implicit call-stacks are to regular implicit parameters.

 ---

 I'm actually more concerned about the fact that `-ffull-laziness` changes
 the behavior. Does this mean we're caching the first call-stack? That
 would be completely wrong; it would be wrong for any implicit parameter.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11383#comment:2>
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