I've isolated the issue to the handling of if defined on multi-argument macros. I took a crack at interpreting the cpphs source for this and I think it may be a bug in the conversion of defined expressions here in Language.Preprocessor.CppIfdef here: convert "defined" [arg] = case lookupST arg st of Nothing | all isDigit arg -> return arg Nothing -> return "0" Just (a@AntiDefined{}) -> return "0" Just (a@SymbolReplacement{}) -> return "1" Just (a@MacroExpansion{}) -> return "1" It looks like it will macro expand the contents of a defined expression which isn't what GCC does. I don't know if GCC is wrong or if using parameterized macros within if defined works on single-argument macros. working1.hs: {-# LANGUAGE CPP #-} #define EXAMPLE_MACRO(arg) (\ arg) #if defined(EXAMPLE_MACRO) #endif preprocess it (it works!): $ cpphs --cpp working1.hs -o $tempfile $ ifdef works on multiple-argument macros. working2.hs: {-# LANGUAGE CPP #-} #define EXAMPLE_MACRO(arg1,arg2) (\ arg1 > arg2) #ifdef EXAMPLE_MACRO #endif preprocess it (it works!): $ cpphs --cpp working2.hs -o $tempfile $ if defined fails on multi-argument macros. broken2.hs: {-# LANGUAGE CPP #-} #define EXAMPLE_MACRO(arg1,arg2) (\ arg1 > arg2) #if defined(EXAMPLE_MACRO) #endif preprocess it (it fails!): $ cpphs --cpp broken2.hs -o $tempfile cpphs: macro EXAMPLE_MACRO expected 2 arguments, but was given 0 $ I've posted a StackOverflow question to see if any of them know if this is undefined behavior: http://stackoverflow.com/questions/34709769/is-cpphs-wrong-or-is-the-behavio... If it is undefined behavior we should stop relying on it in GHC sources. Either way the behavior is inconsistent with GCC which complicates things. Best, Alain On Sun, Jan 10, 2016 at 2:04 PM Alain O'Dea <alain.odea@gmail.com> wrote:
Hi Malcolm:
cpphs is under consideration as a replacement for GCC's C preprocessor in the GHC toolchain: https://ghc.haskell.org/trac/ghc/wiki/Proposal/NativeCpp
GHC 7.10.3's build fails when cpphs is used as the C preprocessor (--with-hs-cpp=cpphs --with-hs-cpp-flags="--cpp").
It runs into this error when preprocessing libraries/base/GHC/Natural.hs:
cpphs: macro MIN_VERSION_integer_gmp expected 3 arguments, but was given 0
I've reproduced this issue on Ubuntu 14.04 x86-64 and SmartOS 15.3.0 x86-64.
Interestingly the error seems to arise only when preprocessing Natural.hs while the autogenerated cabal-macros.h is present. Removing that include from the cpphs flags leads to a clean preprocessing run.
I have more details of this investigation here: https://gist.github.com/AlainODea/bd5b3e0e6f7c4227f009
Is this a bug?
Best, Alain