Simpler support for isolated/sandboxed builds

[duncan]

Cabal/cabal-install allows setting the install prefix and the package database. Together these allow creating isolated builds. This is clearly not sufficiently easy to use as there are now two projects that provide wrappers over cabal-install to provide more direct support for this use case:

•  http://github.com/creswick/cabal-dev/blob/master/README.md
•  http://www.haskell.org/haskellwiki/Capri

The task is to investigate what user behaviour is desired and to integrate this in the best way with Cabal/cabal-install.

This is probably related to:

• ticket #556 about local config files (e.g. persistently setting some sandbox flag)
• ticket #734 about supporting multiple local .cabal files
• ticket #524 about supporting file, directory and url targets for packages and .cabal files

[nathan.collins]

I just attempted my first "sandboxed" build with Cabal, and failed, and I agree it could be much easier. After about 2 hours I was still failing, and I came here to see if I had run into a bug. But, trying the Capri package you mention above, I was able to figure out a sandboxed build in about 5 minutes!

Here's where I gave up using cabal (some output elided here and in other examples):

$ ghc-pkg init ./cabal/packages
$ prefix=`pwd`/cabal
$ cd lib/replib-read-only
$ cabal install --prefix $prefix --package-db=$prefix/packages
$ cd ../../src
$ cabal install --prefix $prefix --package-db=$prefix/packages
...
Loading package RepLib-0.3.1 ... linking ... done.
...
Registering trellys-core-0.0...
cabal: trellys-core-0.0: dependency
"RepLib-0.3.1-bd84a6d961076829097d3c4eb1eaee2c" doesn't exist (use --force to
override)
cabal: Error: some packages failed to install:
trellys-core-0.0 failed during the building phase. The exception was:
ExitFailure 1

I wonder if this is a bug, because the failure was after building and linking, and ghc-pkg knows about the package:

$ cd ..
$ ghc-pkg list --package-conf cabal/packages/ -v
using cache: cabal/packages/package.cache
using cache: /home/collins/.ghc/i386-linux-6.12.1/package.conf.d/package.cache
using cache: /var/lib/ghc-6.12.1/package.conf.d/package.cache
cabal/packages/
   RepLib-0.3.1 (RepLib-0.3.1-bd84a6d961076829097d3c4eb1eaee2c)

Now, here's the successful capri version, for comparison:

$ capri bootstrap
$ capri clone syb 'base-3*'
$ capri import lib/replib-read-only
$ capri import src

To be fair, the capri version had one hangup: in my first try I didn't have the

$ capri clone syb 'base-*'

line, and I got

...
Linking dist/build/trellys/trellys ...
<command line>: unknown package: base-3.0.3.2
cabal: Error: some packages failed to install:
trellys-core-0.0 failed during the building phase. The exception was:
ExitFailure 1

in the capri import src step. But, a very similar error is covered in the second example in the Capri documentation (which in total is 12 pages in my browser, and onl y required skimming), so the solution was quickly found.

Comparing the (failed) cabal version with the capri version, they're both simple. The difference is that it took me 2 hours to come up with the cabal version, versus 5 minutes to come up with the capri version. The reason for the time difference is that capri's (short) documentation is all about doing what I wanted to do, whereas cabal's (long) documentation has no section devoted to sandboxed installs. So, assuming there is an easy way to do this with cabal, it would probably be enough to add a section to the documentation that explained the process for people that want to move from a (working, but not sandboxed) install process like

$ cd lib/replib-read-only
$ cabal install
$ cd ../../src
$ cabal install

to a sandboxed one.

P.S. the code I'm trying to build is a working copy of  https://code.google.com/p/trellys/, in case someone is dying to debug my failed cabal sandboxed build :)