Segfault in a CoreLinted program (and a GHC-generated Core question)

Ömer Sinan Ağacan omeragacan at
Mon Oct 26 23:57:19 UTC 2015

OK, thanks to people at IRC channel(especially @rwbarton) I realized
that my lint calls were not actually running, simply because I wasn't
using -dcore-lint.. I didn't know such a flag exists, and even with
the absence of the flag I'd expect a core lint would work, because I'm
explicitly calling the lint function without checking any flags!

CoreLint is now giving me really awesome diagnostics! Sorry for the noise..

(I'll try to document linter functions or CoreLint module to let the
user know he/she needs this flag!)

2015-10-26 13:43 GMT-04:00 Simon Peyton Jones <simonpj at>:
> |  So my questions are: Am I right in assuming that CoreLint accepted programs
> |  should not segfault?
> Yes.  Modulo unsafeCoerce, and FFI calls.
> | What about internal invariants? Should CoreLint check
> |  for those? Is there any pass that checks for invariants and prints helpful
> |  messages in case of a invariant invalidation?
> Yes; they are documented in CoreSyn, which the data type, and Lint checks them.
> |  As an attempt at debugging the code generated by my plugin, I wrote the
> |  function that is supposed to be generated by my Core pass in Haskell and
> |  compiled with GHC. Generated Core is mostly the same, except at one point it
> |  has an extra lambda directly applied to a void#, something like ((\_ -> ...)
> |  void#). Where can I learn more about why GHC is doing that?
> Show me the code!
> Instead of generating
>     x :: Int# = <some expression that might fail>
> GHC sometimes generates
>     x :: Void# -> Int# = \_ - <some expression that might fail>
> and then calls (x void#), to make any div-zero failures happen at the right place.
> I'm not sure if that is what you are seeing, but we can work it out when you give more detail.
> Simon

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