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

[Viktor Dukhovni]

On Sat, Feb 01, 2020 at 11:36:19AM +0100, Merijn Verstraaten wrote:

> > On 1 Feb 2020, at 10:01, Viktor Dukhovni <ietf-dane at dukhovni.org> 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.