[GHC] #15613: GHCi command, tracing steps of instance resolution for Constraint or expression

[GHC]

#15613: GHCi command, tracing steps of instance resolution for Constraint or
expression
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  GHCi              |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15610            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I think you might want something more like `simplifyDefault` or
 `solveWanteds`, both exported by `TcSimplify`.

 Mostly these functions are used ''within'' the typechecker, but the
 pattern match overlap checker uses the constraint solver in
 `Check.tyOracle`.

 A significant factor is whether you want any "givens"; that is, do you
 want to explain why a constraint fails to be solved under a GADT match, or
 in a function with a given context.

 Once you get deeper into the constraint solver, solving one step of each
 individual constraint, you need the `TcS` monad, which holds the inert
 set... I'm not sure you want to get into that.  Better to tell the
 constraint solver to try to solve, and see what is left over: it'll return
 (a) any unsolved constraints and (b) evidence bindings that amount to a
 specification of the exact proof that it came up with; i.e. the tree you
 want to display.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15613#comment:9>
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