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
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