Re: cabal design

On Mon, Aug 15, 2005 at 08:52:18AM -0700, Isaac Jones wrote:

Frederik Eaton writes:

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1) Cabal files can have different names, yet only one can exist in a directory. I've been told that the reason for the former is that Isaac wants to support building multiple packages in a directory, but currently the feature isn't implemented, so only one cabal file is allowed.

Certainly, it would be a nice feature, but more importantly it seems that it would require some API changes or additions which should be made sooner rather than later. Is there a document describing how it would work? Is there a roadmap, a schedule for implementation?

I'm not so sure that the change is very big at all... it should just be a matter of adding a feature to allow libraries to register themselves "in-place" or something... then compute the dependencies between the packages, build them in order, install the libraries you'll need in-place, then compile the programs.

The problems come up where a package might need to actually be in the install location in order to work. Hopefully that's not too common.

This sounds rather vague. For instance, how does it interact with configure/make based packages? What do you think about putting a TODO list linking to designish documents on the wiki, so that people can flesh these plans out and comment on them there?

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3) The way that a 'cabal' package with a 'configure' script should be built is not easy to determine. There are some very useful programs (http://toastball.net/toast/) which build and manage packages of all kinds automatically, by running 'configure' with a local prefix, etc. How should these tools be extended to work with 'cabal' packages? With some 'cabal' packages, the user is supposed to run 'configure' first, and then 'runhaskell Setup.hs'. With other 'cabal' packages, the user is supposed to just run 'runhaskell Setup.hs', and this will run 'configure' automatically. Is there a right way? Is there a way to distinguish the two types of packages automatically?

What packages require you to run configure by hand? That is indeed incorrect in the sense that it doesn't conform to the cabal interface. If the packager is doing that in order to alter the .cabal file, then that's a problem... the correct way to do this is covered in the manual, and configure has built-in support in cabal.

I don't know of any packages off-hand. I think my friend who wrote "toast" may have found one, but he may also have just liked 'configure' better. Anyway, it's good to know that there is a standard. It might be good for the manual to mention explicitly that configure/make are always run by Setup.hs and not vice-versa.

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4) Cabal's inability to let users specify package.conf-s other than the user or global package.conf

I think the gentoo guys were going to implement this for us; in any case, it's a subset of 1). This should be pretty easy; the only question is whether we should have a --in-place or a --package-conf=... Where in-place would choose a "standard" one (./packages.conf or something). The reason I'd prefer the former is that not all systems _have_ an idea of a package.conf file, so --package-conf= would make no sense for most compilers. I try to avoid that. The reason for the later would be if there's some use-case that --in-place does not work for.

Is it really a subset of (1)? I don't think it is. I think a --package-conf= would be very nice. If it is not supported by the compiler, then an error should be generated.

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, and ghc's refusal (and that of other compilers) to let users specify paths for things via environment variables, (snip) To lag behind 'gcc' and 'autoconf' is not acceptable.

GCC lets you specify things via environment variables? I've never seen that... if ghc were to adopt this kind of behavior, I hope it wouldn't be the default; as in 'make', it should require an extra flag like -e.

Mostly CPATH and LIBRARY_PATH. Perhaps, variants for other languages.

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5) It seems that there should be a way to build multiple packages which depend on each other, in such a way as that the first package doesn't have to be installed for the second package to be built. In other words, perhaps something like a dummy installation as in #4 is called for. I have seen mention that support for building multiple packages is planned, but how? When?

This is the same as 1.

Well, not if they aren't in the same directory. Again, we need to have more explicit details about how things are planned to work.

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What stage of implementation are we in?

We're in the stage of "Isaac has been too busy writing papers and book reviews and needs to get working." I'm always happy to get patches, though.

OK, correct me if I'm wrong but I think we need to tell people what kind of patches to write, and ask them to write them, to get things moving.

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6) I think it should be easy to use 'cabal' for development; however, when I am building a package with multiple executables, every 'build' seems to re-link each executable, regardless of whether it needs re-linking or not, which is quite slow.

I'm not sure why ghc --make re-links executables every time.

For libraries, I think we could use support from ghc to tell whether we need to re-link the library; ghc goes through and skips stuff that doesn't need to get built, and then we link everything in the library; if ghc could somehow let us know that nothing needed to get built, that would be very helpful; otherwise, someone has to write the logic to go through and check it all just like ghc does. This code should be out there somewhere.

Isn't there a way to get ghc to emit Makefile fragments which solves this problem? Not that solving it in ghc wouldn't be good as well.

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I'm not sure how much of this Isaac has planned out on his own and is busy implementing, vs. how much is waiting for someone to write a design document for.

You're always welcome to submit patches and suggestions on how to do stuff, and I and other folks here will be more than happy to engage you. I've been a little too distracted with other stuff, and keep hoping that GHC 6.4.1 will appear so I can release cabal 6.2 and get hacking on other stuff. I should just make a 'stable' branch or something.

Finally, thanks for all of your great suggestions. We need feedback from as many different users as possible. We've spent a long time guessing about what folks might need, and now that cabal is in the wild, we're getting good feedback about what they actually need :)

You're welcome. Frederik