GHCi panics with 'initTc:LIE' while :stepping on code with funny types

[mnislaih]

This is due to runtime type reconstruction not dealing correctly with type functions.

I managed to come up a minimal example.

{-# LANGUAGE TypeFamilies #-}

type family Id x
type instance Id Int = Int
type instance Id Bool = Bool

class Convert x y where convert :: x -> y
instance Convert x x where convert = id

f :: Convert a (Id a) => a -> Id a
f x = convert x

g :: Convert a (Id a) => a -> Id a
g x = let x' = f x in x'
}
{{{
[1 of 1] Compiling Main             ( ../code/debuggerExamples/lie.hs, interpreted )
Ok, modules loaded: Main.
*Main GOA> :step g False
:step g False
Stopped at ../code/debuggerExamples/lie.hs:15:0-23
_result :: Id a = _
14  g :: Convert a (Id a) => a -> Id a
15  g x = let x' = f x in x'
    ^^^^^^^^^^^^^^^^^^^^^^^^
[../code/debuggerExamples/lie.hs:15:0-23] *Main GOA> :step
:step
ghc: panic! (the 'impossible' happened)
  (GHC version 6.10.1 for i386-apple-darwin):
	initTc:LIE

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
}}}

[mnislaih]

Fixed, please merge

Tue Mar  3 11:37:06 PST 2009  pepe iborra <mnislaih@gmail.com>
 * Fix #3067: GHCi panics with 'initTc:LIE' while :stepping on code with funny types

 The problem is that calls to boxyUnify would panic if the types involved
 contained type functions. 
 It looks like one should wrap these calls with getLIE, although I don't
 really know what I am doing here



   M ./compiler/ghci/RtClosureInspect.hs -2 +2

[igloo]

Merged.