[GHC] #13201: Type-level naturals aren't instantiate with GHCi debugger

[GHC]

#13201: Type-level naturals aren't instantiate with GHCi debugger
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        Reporter:  konn              |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      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 rwbarton):

 I took a quick look at this, thinking the issue would be that, in `foos`,
 the information of the specific type `n` is unavailable at runtime. But
 apparently it's not so simple, since if you write an ordinary
 parametrically polymorphic function like
 {{{#!hs
 myFun :: [a] -> [a]
 myFun l = l ++ reverse l
 }}}
 and step into `myFun`, ghci is still capable of displaying the specific
 type of `l`. So, my conclusion is that how this works is a mystery.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13201#comment:2>
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