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

[GHC]

#13201: Type-level naturals aren't instantiated 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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Changes (by akio):

 * cc: akio (added)


Comment:

 Here is a test case that doesn't use type-level naturals:

 {{{#!hs
 {-# LANGUAGE StandaloneDeriving #-}
 data Foo a = Foo

 deriving instance (Show a) => Show (Foo a)

 fooSucc :: Foo a -> Foo [a]
 fooSucc Foo = Foo

 foos :: Foo a -> [Foo [a]]
 foos f = loop 5
   where
       loop 0 = []
       loop n = fooSucc f : loop (n - 1)

 main :: IO ()
 main = print $ foos (Foo :: Foo Int)
 }}}


 By no means I'm an export on this, but as far as I know, the GHCi debugger
 tries to re-construct the type information as much as possible by
 inspecting the runtime representation (closures) of the values. This means
 it cannot distinguish between different types with the exact same
 representation. For example, it cannot distinguish a newtype from its
 content type, nor can it determine a phantom type parameter of a data
 type.

 In this example, since `n` is only used as a phantom parameter of the
 datatype `Foo`, the GHCi debugger has no way of reconstructing it at
 runtime, I think.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13201#comment:4>
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