cabal install --only

[Duncan Coutts]

On Fri, 2013-03-15 at 12:47 +0100, Mikhail Glushenkov wrote:
> Hi,
> 
> On Fri, Mar 15, 2013 at 8:14 AM, Andres Löh <andres at well-typed.com> wrote:
> >
> > No, please do not remove it. Running install without --only causes all sorts
> > of things to happen, because install is the magic command. The --only flag
> > is for use after you have already run build and really only does the
> > installation, i.e. It behaves comparable to Setup install.
> 
> Yes, I see from the code what it does, it's just that I think that the
> option is obscure and very rarely used. It's not shown in the --help
> output, though, so I guess it's not a big problem.

As other people have pointed out it's rather useful.

The reason it's there is because our UI is confused. It's a wart that
needs fixing not by removing it, but by removing the need for it.

When we had the old "runhaskell Setup {configure,build,install}"
interface then it was clear that the commands acted only on the local
package and only did one step of the process: you had to have done the
other steps first.

When we moved to cabal-install, we wanted automation and a "do
everything" command. So the install command will configure, build and
install. But there isn't a proper dependency mechanism and there isn't a
proper tracking of configuration state. So install always reconfigures,
loosing any configuration you did previously with cabal configure. So
that means if you do: cabal {configure --blah,build,install} then you
loose that --blah config. So because of that one does sometimes want a
--only. It's hidden because it's not official UI.

So what would a proper UI look like here? I think one should consider it
in the new context of sandboxes, or more generally the idea that in
future the cabal command line UI does not act on individual packages in
isolation but on packages in the context of an environment containing a
set of source packages.

So when I'm in an environment and I say configure on a package, that
should save semi-persistently the config settings I give (and do package
dependency analysis, if necessary it should (re)configure the other
source packages in the environment that this one depends on). The build
and install commands should work in a make-like fashion in that they
have to be able to discover/check which packages are up to date
(including if they are configured already, and if we already had an up
to date InstallPlan). So install --only is redundant because install
will do the right thing if you've already configured and built.

And that's of course the reason we still have --only, because we don't
have a proper make-like system for checking if packages are already
configured or built etc.

Duncan