[Haskell-cafe] Cabal, lib and exe in one

Simon Marlow simonmarhaskell at gmail.com
Wed May 2 05:08:41 EDT 2007

Duncan Coutts wrote:
> On Tue, 2007-05-01 at 22:29 +0100, Magnus Therning wrote:
>>> So if foo.hs is in test-src and Foo/Bar.hs is in src then I think you
>>> just need:
>>> hs-source-dirs: test-src, src
>> No, that's not enough, I also have to add the following lines to make
>> the executable compile and link:
>>   extensions: ForeignFunctionInterface
>>   c-sources: csrc/ptrace.c
>> That is, I end up compiling the library a second time!  Can't I get the
>> executable to link against the library that was just created?
>> I was just expecting to not have to repeat myself in the cabal file.
>> Not such a strange thing to expect from a build system, I think :-)
> Yes this is an interesting question about what it means to have programs
> in the same cabal package as an executable.
> Currently having a executable and a library inside a cabal package is
> not the same thing as having a library package and separate package that
> contains only that executable. The difference is that when the
> executable is in the same cabal package it merely has access to the same
> modules, it doesn't 'depend' on that library package exactly. So for
> example it can access modules which are not exposed by the library and
> indeed it can compile those same modules with completely different build
> flags. So currently those modules will be built twice.
> It's not clear to me that this is the right meaning, or indeed that we
> should allow multiple entries in a single .cabal file. I think it might
> be better to just have multiple .cabal files (possibly in the same
> directory). Then we could be explicit and state that an executable
> depends on the library or if we want to use different build flags, or
> use modules that are not exposed by the lib then we can do that and only
> in that case do we build those modules twice.

Right at the front of the Cabal docs it says:

"However having both a library and executables in a package does not work very 
well; if the executables depend on the library, they must explicitly list all 
the modules they directly or indirectly import from that library."

IMO we shouldn't allow both a library and an exe in the same package.  I think I 
argued against this originally, and my understanding is that doing this is 
deprecated, although perhaps not visibly enough.  Whenever the question of what 
to do about lib+exe packages arises, the discussion tends to spiral out of 
control into what we should do about collections of packages in general.

For now, the simple story is that each package should have either a single 
library or a single executable (even multiple executables in a package is 
questionable; if they share some code it shoud be in a package).


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