Re: [Haskell-cafe] Error trying to build haskell-dbus on FreeBSD

1 Feb 2020

      On Sat, Feb 01, 2020 at 11:36:19AM +0100, Merijn Verstraaten wrote:
...
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On 1 Feb 2020, at 10:01, Viktor Dukhovni  wrote:
The package developer.  Neither GHC nor Clang are at-fault for invalid
CPP macro syntax.
This isn't the package developers fault. Standard Haskell just cannot
be properly preprocessed by an ISO C preprocessor. As the definition
of valid tokens just aren't compatible.
Well, my take was the package developer using "CPP" is expected to
produce CPP-compatible input.  But if Haskell is expected to provide a
more liberal dialect of CPP, then indeed perhaps the issue is with the
Haskell build using Clang's CPP.
...
GHC's use of the CPP has long (maybe since forever?) been incompatible
with CPP as standardised in the ISO C spec, and its relies on the
-traditional flag to make GCC use it's pre-ISO C implementations. This
flag isn't necessarily portable across C compilers and I recall this
breaking things in the past.
Indeed the -traditional flag shows the difference, only GCC's "cpp"
tolerates the C-incompatible tokens, Clang does not.

    $ for cpp in gcc{7,8,9} clang{37,39,60,70}; do printf "--- CPP: %s\n" "$cpp"; printf "#define foo a' = b\nfoo\n" | $cpp -traditional -Werror -E - 2>&1; printf "EXIT: $?\n"; done | egrep -v '^# [0-9]|^$'
    --- CPP: gcc7
    a' = b
    EXIT: 0
    --- CPP: gcc8
    a' = b
    EXIT: 0
    --- CPP: gcc9
    a' = b
    EXIT: 0
    --- CPP: clang37
    <stdin>:1:14: error: missing terminating ' character [-Werror,-Winvalid-pp-token]
    #define foo a' = b
                 ^
    a' = b
    1 error generated.
    EXIT: 1
    --- CPP: clang39
    <stdin>:1:14: error: missing terminating ' character [-Werror,-Winvalid-pp-token]
    #define foo a' = b
                 ^
    a' = b
    1 error generated.
    EXIT: 1
    --- CPP: clang60
    <stdin>:1:14: error: missing terminating ' character [-Werror,-Winvalid-pp-token]
    #define foo a' = b
                 ^
    a' = b
    1 error generated.
    EXIT: 1
    --- CPP: clang70
    <stdin>:1:14: error: missing terminating ' character [-Werror,-Winvalid-pp-token]
    #define foo a' = b
                 ^
    a' = b
    1 error generated.
    EXIT: 1

My own build of GHC for FreeBSD has settings:

    [("GCC extra via C opts", " -fwrapv -fno-builtin"),
     ("C compiler command", "gcc"),
     ("C compiler flags", ""),
     ("C compiler link flags", " -fuse-ld=lld"),
     ("C compiler supports -no-pie", "YES"),
     ("Haskell CPP command","gcc"),
     ("Haskell CPP flags","-E -undef -traditional"),
     ...
    ]

It seems that the ports system elects clang, leading to the reported
issue.  I see that FreeBSD ports also include:

    hs-cpphs-1.20.8_7   Liberalised re-implementation of cpp, the C pre-processor

Which means that either hs-cpphs needs to be a pre-requisite (built by
the boostrap GHC compiler?), a co-requisite (assuming GHC itself does
not break with clang's CPP), or the configuration needs be tweaked to
use "gcc" as the Haskell CPP preprocessor, even when clang is the
compiler.

-- 
    Viktor.