#13201: Type-level naturals aren't instantiated with GHCi debugger -------------------------------------+------------------------------------- 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: | -------------------------------------+------------------------------------- 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. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13201#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler