Doesn’t Play Nice With Others

For the last few weeks, I’ve been looking into making a Printer Module in Haskell. I must say, it’s been a pretty miserable experience. Not the Haskell part, that was ok. No, my issue is more basic. It seems that Haskell doesn’t like to share.

My plan was to build a module in Haskell to do the printer logic, then link that module as a library into C, which will be imported by the Core as normal. A preliminary look about the internet confirms that this is supported behavior. There are a few trivial examples peppered throughout the internet; so I set to work, confident that this was a solvable problem.

Giving Cabal A Shot

Cabal is Haskell’s package management program. In addition to this, it serves as Haskell’s answer to make. With a simple call to:

cabal init

You are presented with a series of questions about your package. After filling out the form (and selecting library), Cabal creates a Setup.hs file. Calling:

runhaskell Setup.hs configure runhaskell Setup.hs build

…produces a .a library for your package. Success, right? Unfortunately when you try to load that you get a linker error stating something to the effect of “can not be used when making a shared object; recompile with -fPIC“. After hours of research I have determined that this is because the Haskell’s libraries have not been compiled using the -fPIC, which prevents them from being used with a static library.

Trying GHC

The Glasgow Haskell Compiler can be used to compile libraries directly. Having given up on cabal, I decided to try to cut out the middle man and use GHC. After much tinkering, I came up with a Makefile that worked, which I will preserve here for posterity:

COMPILER=ghc HS_RTS=HSrts-ghc7.6.3 OUTPUT=dmp_printer_module.so all: "${COMPILER}" --make -no-hs-main -dynamic \ -l${HS_RTS} -shared -fPIC dmp_printer_module.c \ DmpPrinterModule.hs -o ${OUTPUT}

This makefile compiles any required Haskell scripts, as well as a C “glue” source file that initializes and finalizes the Haskell environment. More on what goes in that file can be found in the GHC documentation.

Cool, good to go, right? Wrong.

Couldn’t Find That Dyn Library

So, I started adding things to the module to make sure it doesn’t break. After adding some dependencies, and trying to recomplie, I start seeing this error:

Could not find module `[SOME_MODULE]' Perhaps you haven't installed the "dyn" libraries for package `[SOME_PACKAGE]'?

After more hours of research, it turns out that for a module to be used in a shared library, it must be compiled as one. Seems logical, but that would imply that all module developers have to go through this nightmare. And the developers of any dependencies they use have to have done so. And so on…

Since even Prelude hadn’t done so, I set off to figure this out. After poking about, it turns out that Debian provides a package ghc-dynamic, which provides the dyn libraries from Base. I installed it, and things were checking out. However, the dependencies I was using still did not work.

After some more reasearch, I found a suggestion that I re-install all my Cabal packages using the --enable-shared flag, which would provide me with my dyn libraries. I gave it a shot, but since my dependencies’ dependencies hadn’t done so, I got the same errors.

Some more research suggested that I could delete the .ghc folder in my home folder, then re-install all Cabal packages. This would force them to rebuild. However, I encountered the same issues.

The Man In Black Fled Across The Desert…

I’m beginning to feel a bit like Roland, ceaselessly chasing after the Dark Tower. Every time I get there, my journey starts over again. I clear one roadblock, and there’s another there waiting for me.

It seems like there isn’t any real interest in calling Haskell from C, and I must say that I am extremely disappointed. Calling C from Haskell works great, but when asked to share its toys with C, Haskell takes its ball and goes home.

I’m sure it’s possible to do meaningful work in Haskell, and call that from C. However, the amount of work I would have to do to attain that goal is not something I’m willing to accept. For this reason I am shelving Haskell for the time being. Maybe I’ll pick it again for some other project, but it’s not a good fit for DMP Photo Booth.

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