[GHC] #14995: QuantifiedConstraints: Incorrect pretty printing

[GHC]

#14995: QuantifiedConstraints: Incorrect pretty printing
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  GHCi              |              Version:  8.2.2
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints, wipT2893
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 This is like #14897.  It'l be fine if you say `:info k`.  But with `:t k`
 you are asking GHC to infer the type of the expression `k`.  So it
 instantiates `k` to get the constraint
 {{{
 [W] ((C a1 => B a1 b1))     -- A quantified constraint
 }}}
 As usual `a1` and `b1` are unification variables.

 To solve a quantified constraint we create an implication constraint
 {{{
 forall {}. C a1 => [W] B a1 b1
 }}}
 Now we are stuck, so we try to generalise.  GHC tries to find a simple
 constraint to generalise over, and `B a1 b1` does the job.

 GHC ''never'' infers a quantified constraint.  So `:t` will never display
 a quantified constraint.

 I don't know whether others trip over this `:type` vs `:info` distinction,
 but you certainly do!   Maybe `:type` should work only on identifiers, and
 be identical to `:info`.  We could have something else for arbitrary
 expressions.  Or maybe `:type` should behave non-uniformly for
 identifiers.

 Opinions welcome.  But as it stands it's by-design.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14995#comment:2>
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