Need help with debugging "Impossible case alternative"

Jan Stolarek jan.stolarek at
Fri May 10 12:52:24 CEST 2013

I'm working on modifying comparison primops so that they return Int# instead of Bool. I've been 
stuck with one bug for about two weeks now and I just reached a point where I don't have any more 
ideas what to do. I would appreciate any hints or ideas.

So far I changed the primops to return Int# and created a module GHC.PrimWrappers that contains 
wrappers around the new primops: 

(>#) :: Int# -> Int# -> Bool
(>#) a b = tagToEnum# (a >$# b)  -- (>$#) :: Int# -> Int# -> Int#

I also made changes to base, integer-gmp, primitive and ghc-prim libraries

The problem is that many compiled programs produce "Impossible case alternative" message when they 
are run. This is generated by missingAlt function, which is called by rebuildCase function on 
line 1785 in simplCore/Simplify.lhs. I modified the message displayed by missingAlt to get 
information about the scrutinee, binder and case alternatives:

missingAlt env scrut@(LitAlt _) case_bndr alts cont
  = WARN( True, ptext (sLit "missing LitAlt: ") <+> ppr case_bndr )
    return (env, mkImpossibleExpr_REMOVE_ME (contResultType cont) (showSDoc unsafeGlobalDynFlags $ 
ptext (sLit "missing LitAlt : ") <+> ppr scrut <+> ppr case_bndr <+> ppr alts))

Here's an example error message:

Impossible case alternative: missing LitAlt :  1 wild [(GHC.Types.False, [], GHC.Types.False),
                          (GHC.Types.True, [], x)]

If I understand correctly this means that the case tries to scrutinize integer 1 against True and 
False, which obviously fails (surprisingly, compiling with -dcore-lint and -dcmm-lint doesn't 
detect any problems). My theory is that this integer is supposed to be a tag of a Bool data type 
(in the example message this would be True) and the call to tagToEnum# somehow got eliminated or 
floated out of the case scrutinee. Is that a reasonable theory? Is there a transform that could 
perform such floating of tagToEnum# and if so how to prevent it?

I am unable to create a minimal code example that would cause this problem. I noticed that the 
problem happens on every attempt to print a floating point number (this causes tens of tests in 
the testsuite to fail). I extracted all the code responsible for converting Double to String from 
the GHC.Float and GHC.Show modules so that I have a self-contained program (except the call to 
print). Surprisingly that extracted code does not produce the error message. I'm clueless and 
will really appreciate any ideas.


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