Debugging inside the typechecker knot

[Ryan Scott]

Recently, I've found myself wanting to make changes to code that works
inside code where TyCons are knot-tied [1]. But I've found myself unable to
make much progress, since my ability to debug this code is severely limited
by the fact that I can't seem to trace anything. That is, if I try tracing
anything that comes from a knot-tied TyCon, GHC will go into an infinite
loop! This prevents me from learning *anything* about the types floating
around in the code I want to fix, which is hugely annoying.

How do others work around this problem? There must be *some* way to glean
information about the types inside a typechecker knot, since others seem to
be able to work within this space. How do you do it? And will you consider
sharing your secrets?

Ryan S.
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[1] See Note [Type-checking inside the knot] at
http://git.haskell.org/ghc.git/blob/884bd21a917f607b5a44e038e06f78d0b765ea63:/compiler/typecheck/TcHsType.hs#l1087
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