I want to emphasize that cpphs is actively maintained as it's pointed out in https://ghc.haskell.org/trac/ghc/wiki/Proposal/NativeCp. The Agda team has found some cpphs bugs which have been *quickly* fixed by cpphs's author, Malcolm Wallace. Yes. It was not my intention to imply in any way that cpphs suffers from maintanance issues.

Unfortunately I have not been able to track down the problem mentioned by Janek Stolarek. Yes, and that's what gets me worried. I suppose the problem was somehow related to my locale settings although we were unable to track down the cause. I also recall someone else reported being affected by the same problem. So, until that problem is solved I would say it is a blocker as it would essentialy make development of GHC impossible for some people.

Janek

On 6 May 2015 at 06:38, Jan Stolarek wrote:

...

One thing to keep in mind is that while cpphs will solve some problems of system-cpp, it will also bring problems of its own. I, for one, have run into problems with it when installing Agda. There is a very long thread here:

https://lists.chalmers.se/pipermail/agda/2014/006975.html

and twice as more on my private inbox. We've reached no conclusion about the cause and the only solution was to use system-cpp.

Regarding licensing issues: perhaps we should simply ask Malcolm Wallace if he would consider changing the license for the sake of GHC? Or perhaps he could grant a custom-tailored license to the GHC project? After all, the project page [1] says: " If that's a problem for you, contact me to make other arrangements."

Janek

[1] http://projects.haskell.org/cpphs/

Dnia środa, 6 maja 2015, Herbert Valerio Riedel napisał:

...

Hello *,

As you may be aware, GHC's `{-# LANGUAGE CPP #-}` language extension currently relies on the system's C-compiler bundled `cpp` program to provide a "traditional mode" c-preprocessor.

This has caused several problems in the past, since parsing Haskell code with a preprocessor mode designed for use with C's tokenizer has caused already quite some problems[1] in the past. I'd like to see GHC 7.12 adopt an implemntation of `-XCPP` that does not rely on the shaky system-`cpp` foundation. To this end I've created a wiki page

https://ghc.haskell.org/trac/ghc/wiki/Proposal/NativeCpp

to describe the actual problems in more detail, and a couple of possible ways forward. Ideally, we'd simply integrate `cpphs` into GHC (i.e. "plan 2"). However, due to `cpp`s non-BSD3 license this should be discussed and debated since affects the overall-license of the GHC code-base, which may or may not be a problem to GHC's user-base (and that's what I hope this discussion will help to find out).

So please go ahead and read the Wiki page... and then speak your mind!

Thanks, HVR

[1]: ...does anybody remember the issues Haskell packages (& GHC) encountered when Apple switched to the Clang tool-chain, thereby causing code using `-XCPP` to suddenly break due to subtly different `cpp`-semantics?

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On 2015-05-06 at 13:38:16 +0200, Jan Stolarek wrote: [...]

Regarding licensing issues: perhaps we should simply ask Malcolm Wallace if he would consider changing the license for the sake of GHC? Or perhaps he could grant a custom-tailored license to the GHC project? After all, the project page [1] says: " If that's a problem for you, contact me to make other arrangements."

Fyi, Neil talked to him[1]: | I talked to Malcolm. His contention is that it doesn't actually change | the license of the ghc package. As such, it's just a single extra | license to add to a directory full of licenses, which is no big deal. [1]: http://www.reddit.com/r/haskell/comments/351pur/rfc_native_xcpp_for_ghc_prop...

That doesn't mean those people don't exist. Maybe they do but are too afraid to speak up (due to corporate policy or whatever). On Thu, May 7, 2015 at 10:41 PM, Malcolm Wallace wrote:

I also note that in this discussion, so far not a single person has said that the cpphs licence would actually be a problem for them.

Regards, Malcolm

On 7 May 2015, at 20:54, Herbert Valerio Riedel wrote:

...

On 2015-05-06 at 13:38:16 +0200, Jan Stolarek wrote:

[...]

...

Regarding licensing issues: perhaps we should simply ask Malcolm Wallace if he would consider changing the license for the sake of GHC? Or perhaps he could grant a custom-tailored license to the GHC project? After all, the project page [1] says: " If that's a problem for you, contact me to make other arrangements."

Fyi, Neil talked to him[1]:

| I talked to Malcolm. His contention is that it doesn't actually change | the license of the ghc package. As such, it's just a single extra | license to add to a directory full of licenses, which is no big deal.

[1]: http://www.reddit.com/r/haskell/comments/351pur/rfc_native_xcpp_for_ghc_prop...

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Perhaps it makes sense to scan hackage to find all the different CPP idioms that are actually used in Haskell code, if it is a small/well-defined set it may be worth writing a simple custom preprocessor. Alan On Wed, May 6, 2015 at 3:03 PM, Mikhail Glushenkov < the.dead.shall.rise@gmail.com> wrote:

On 6 May 2015 at 14:25, Austin Seipp wrote:

...

2) The lexing rules for C and Haskell simply are not the same in general.

One area where this is irritating is that it makes it impossible to use Haskell multiline strings together with CPP. _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

2015-05-06 16:21 GMT+02:00 Bardur Arantsson :

+1, I'll wager that the vast majority of usages are just for version range checks.

The OpenGL-related packages used macros to generate some binding magic (a "foreign import" plus some helper functions for each API entry), not just range checks. I had serious trouble when Apple switched to clang, so as a quick fix, the macro-expanded (via GCC's CPP) sources had been checked in. :-P Nowadays the binding is generated from the OpenGL XML registry file, so this is not an issue anymore.

If there are packages that require more, they could just keep using the system-cpp or, I, guess cpphs if it gets baked into GHC. Like you, I'd want to see real evidence that that's actually worth the effort/complication.

Simply relying on the system CPP doesn't work due to the various differences between GCC's CPP and the one from clang, see e.g. https://github.com/haskell-opengl/OpenGLRaw/issues/18#issuecomment-31428380. Ignoring the problem doesn't make it go away... ;-) Note that we still need CPP to handle the various calling conventions on the different platforms when the FFI is used, so it's not only range checks, see e.g. https://github.com/haskell-opengl/OpenGLRaw/blob/master/OpenGLRaw.cabal#L588.

On Fri, May 8, 2015 at 12:12 PM, Dan Doel wrote:

vector generates a considerable amount of code using CPP macros.

And with regard to other mails, I'm not too eager (personally) to port that to template Haskell, even though I'm no fan of CPP. The code generation being done is so dumb that CPP is pretty much perfect for it, and TH would probably just be more work (and it's certainly more work to write it again now that it's already written).

Incidentally, if we really want to pursue the "get rid of CPP by building it into the GHC distro"... In recent years there've been a number of papers on "variational lambda-calculi"[1] which essentially serve to embed flag-based preprocessor conditionals directly into the language itself. One major benefit of this approach is that the compiler can then typecheck *all* variations of the code, rather than only checking whichever particular variation we happen to be compiling at the time. This is extremely useful for avoiding bitrot in the preprocessor conditionals. ...If we were to try and obviate the dependency on CPP, variational typing seems like a far more solid approach than simply reinventing the preprocessing wheel yet again. (The downside, of course, is making the Haskell spec significantly more complex.) [1] e.g., http://dl.acm.org/citation.cfm?doid=2398856.2364535 -- Live well, ~wren

On Fri, May 8, 2015 at 7:40 PM, Donn Cave wrote:

But fatal if compilation is conditional on something that affects the ability to type check, am I right? Such as different compilers or versions of same compiler.

Not per the abstract (paper itself seems to be paywalled). They had an earlier work with that issue, the linked one is about how to be robust in the face of such conditionals. -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

On Sat, 09 May 2015 00:20:25 +0200, wren romano wrote:

In recent years there've been a number of papers on "variational lambda-calculi"[1] which essentially serve to embed flag-based preprocessor conditionals directly into the language itself. : [1] e.g., http://dl.acm.org/citation.cfm?doid=2398856.2364535

Or, without paying: http://web.engr.oregonstate.edu/~walkiner/papers/icfp12-variational-type-err... Best regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/

On Wed, May 6, 2015 at 9:05 AM, Alan & Kim Zimmerman wrote:

Perhaps it makes sense to scan hackage to find all the different CPP idioms that are actually used in Haskell code, if it is a small/well-defined set it may be worth writing a simple custom preprocessor.

Conditional imports are far and away the most commonly used idiom. Second most common, I'd say, is specifying GHC-specific vs compiler-generic implementations of top-level functions (e.g., using GHC.Exts.build or not). For both of these it's sufficient to have the #if construction plus everything needed for the conditional expressions. However, while the #if construction covers the vast majority of use cases, it doesn't cover all of them. Macros are also important. For example, a number of low-level libraries will use macros for things like having assertions which are either compiled as runtime checks, or as nothing, depending on a Cabal flag. Of course, there are plenty of other places where we want to use macros in low-level code, either to force inlining, or to have conditional compilation of (non-top-level) expressions that show up over and over. That these idioms aren't more common is just because there aren't more people working on such low-level code. In theory TH should be able to handle this stuff, but TH is a verbose sledgehammer for these sorts of problems, and using TH means restricting yourself to being GHC-only. -- Live well, ~wren