On 10/31/2012 06:22 PM, Johan Tibell wrote:
Hi all,
Sensible defaults are important to provide a smooth user experience, especially for first-time users. There are a few areas where I think cabal could improve its default behavior. In particular, here are a couple of changes I would like to make:
*cabal build should imply cabal install --only-dependencies * [--snip--] The reasons I've heard for not having build imply installing dependencies is that users might not want packages to be downloaded from the internet, unless they explicitly use cabal install. While that's a fair point, I think it should be an opt-out rather than an opt-in because:
It might be an option to do what Maven (shudder) does here: Have an explicit "offline" mode (switchable by config or cmdline option) which simply blocks any online activity with an error message.
- Users will end up have to install the dependencies anyway and there's typical no other options than installing them from Hackage. - Many users use cabal install instead of cabal build, for the reasons given above, making it a moot point.
Proposal: add a --install-dependencies flag to cabal build that defaults to True.
+1. [--snip--]
*Sandboxing should be used by default*
+1.
This follows the "make it correct, then make it fast" principle. Sandboxing helps with a certain class of build problems, namely problems due to two different builds overwriting each others' dependencies in the shared package database. This problem shows up as a dangerously sounding cabal warning message, mentioning the --force-reinstalls flag.
That, and you end up with broken packages due to no fault of your own (using the default mode). That's simply bad from a usability perspective.
Sandboxing is an imperfect solution to this problem, requiring more rebuilding of packages than strictly necessary. A final solution would involve something like a Nix-style, append-only package database. The Nix solution is being worked on, as part of GSoC, but it's a difficult problem and it requires more wide-reaching changes (e.g. to ghc, ghc-pkg, ghci, and cabal). Since sandboxing is working today (with a few minor tweaks yet to be done) and it can be transparently replaced with a better solution once we have one, I think we should enable it by default in 1.18.
(Drools about the prospect of true Nix-style package database -- hope's to hoping that happens at some point.)
*We should add a cabal run command*
+1.