GHC panic: Template variable unbound in rewrite rule

[mietek]

Installing singletons-1.0 with GHC 7.8.2 on both OS X 10.9.2 and Ubuntu 2014.04 causes a GHC panic:

ghc: panic! (the 'impossible' happened)
  (GHC version 7.8.2 for x86_64-apple-darwin):
        Template variable unbound in rewrite rule

• Build log
• ghc --info

See also GHC #9160.

[jstolarek]

Does this also happen with the latest singletons code from github?

[goldfirere]

Thanks for reporting this here, but if GHC is panicking, then it's properly a GHC bug. Has anyone else run into this? I'm not labeling this as blocked, because perhaps singletons can include a workaround, but first, we'll have to find a way to reproduce...

[mietek]

This seems to be an issue with the optimizer, as the bug only appears with optimization level 2, using the following line in ~/.cabal/config:

optimization: 2

[mietek]

Fixed in GHC 7.8.3.

EDIT: By which I mean—reported as fixed. Apologies for not verifying the fix myself.

[thomie]

Although it's a GHC issue, it's not actually fixed. See: https://ghc.haskell.org/trac/ghc/ticket/9160#comment:10.

This might be related to #96, since the failure happens in the same module (Data.Singletons.Prelude.List).

[goldfirere]

Still keeping this ticket closed, as it's not singletons's problem and there is an easy workaround: use -O1 or -O0.