Understanding ghc-pkg dependencies when not using cabal

[Tim Cuthbertson]

Hi all,

I'm very interested in distributing ghc packages via zero-install [0].
I'm excited about ghc 7.4's ${pkgroot} support, since it means that
many packages could now be made relocatable (as long as they don't
need to know about paths to resources, etc). But I'm having some
trouble understanding exactly what relationships exist between
packages and when packages are needed / used. I have a lot of
questions. Answering all of them would be wonderful, but even just
some at least would be handy too ;)

For an example, lets use ansi-terminal [1] (and assume we're building
it on a unix host)

ansi-terminal depends on `base` and `unix` packages. Do these need to
be present in source form, or just in "compiled package" form? I'm
assuming the latter.
(by "compiled package", I mean the stuff that ends up in e.g
~/.cabal/lib/PACKAGENAME)

When I built ansi-terminal, the package.conf I got out of `cabal
register` had the following field:

depends: base-4.5.0.0-6db966b4cf8c1a91188e66d354ba065e
         unix-2.5.1.0-c004d6a0692bba682bbf2ac7b61d8322

So presumably this is the exact version that was used for compilation.
Obviously the first part is the version, but what is the second part?
Is it the abi-hash of the library that was used at compile time?

If I want to use ansi-terminal in a program that I'm compiling, how
strict are these dependencies? Will a `unix` package of a different
version and the same abi-hash be acceptable? Does it need to be
strictly _newer_ than 2.5.1.0 (and still have the same abi-hash)? Or
is this an exact dependency? If I distribute the `unix` library in
compiled form, how likely is it that a given compiled version of
`unix` will satisfy the requirement?

If I have compiled two libraries that both depend on `unix`, but they
ended up referencing a different abi-hash of `unix` (maybe I compiled
them on a different computer, or on slightly different versions of the
source), what happens when I try to compile an application that
depends on these two libraries? I presume it couldn't work, even if I
had registered both versions of `unix` simultaneously (is this even
possible to do?).

Under what circumstances will the same package source code (e.g
"unix-2.5.1.0") have two different abi-hashes? From [2], it sounds
like only major GHC version changes should affect it. Is the abi-hash
consistent on different platforms (assuming the same major GHC version
is used)?

Can the output of `ghc --abi-hash` (with no arguments) be used as a
"linkage" restriction? (i.e if I have the exact same source and two
versions of ghc with the same abi-hash, will the compiled library
necessarily end up with the same abi-hash in both cases?).

If the `unix` package releases a new version that is compatible with
ansi-terminal, what would I have to do to get this updated code into
my application (that depends on ansi-terminal)?
 - install new version of `unix`, and recompile my application
 - as above, but also rebuilding the current version of
`ansi-terminal` before I rebuild my application (despite ansi-terminal
not actually changing itself)
Perhaps the above question could be rephrased as "if a new version of
`unix` is source-compatible with the previous, what recompilation (if
any) of downstream libraries is / could be required?

I know that ghc compiles applications statically by default. Given
this, I'm a little confused by the lifetime of packages in ghc. Are
packages (in the sense of "the stuff that is listed by `ghc-pkg
list`") only required at compilation time? If I run a precompiled
program that depends on another haskell package which has been
compiled as a shared library, does this mean I need the ghc package to
be registered, or is it sufficient to just have the compiled .so on my
$LD_LIBRARY_PATH?

Apologies for the large number of questions, and thanks for your time.
 - Tim.


[0]: http://0install.net/
[1]: http://hackage.haskell.org/package/ansi-terminal-0.5.5
[2]: http://hackage.haskell.org/trac/ghc/wiki/Commentary/Packages