[GHC] #16089: seq is not cooperating with :sprint in GHCi as expected

[GHC]

#16089: seq is not cooperating with :sprint in GHCi as expected
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           Reporter:  radrow         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  GHCi           |           Version:  8.6.3
           Keywords:  seq sprint     |  Operating System:  Linux
  strictness                         |
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I was playing around with strictness and performed following test:

 {{{#!hs
 Prelude> x = [True, False]
 Prelude> :sprint x
 x = _
 Prelude> x `seq` True
 True
 Prelude> :sprint x
 x = _
 }}}

 I completely don't understand why `x = _` at this moment. `seq` must
 evaluate one level of `x` achieving `x = True : _` to ensure that it is
 not `undefined`, so why is this information lost?

 I am testing it on GHCi version 8.6.3, but it is not reproducible on other
 versions (checked on 8.4.4, 8.2.2 and 7._._).

 I have asked about it on SO, so if this behavior is intended I kindly ask
 for explaination here [https://stackoverflow.com/questions/53898220
 /sprint-and-seq-together-missing-evaluation] to let others know.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16089>
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