cabal v2-install UX is not pleasant

[mitchellwrosen]

Apologies if this is a duplicate ticket, but I hunted around a bit and did not find anything.

The cabal v2-install experience is not very pleasant right now. Here is me trying to install hlint-2.2 from hackage:

> cabal unpack hlint
> cd hlint-2.2

Ok so far.

> cabal v2-install
Resolving dependencies...
Assertion failed
CallStack (from HasCallStack):
  assert, called at ./Distribution/Client/ProjectOrchestration.hs:796:5 in main:Distribution.Client.ProjectOrchestration

??? Baffling. I could try saying more to the tool but I feel I've said it all.

> cabal v2-install hlint
cabal: Ambiguous target 'hlint'. It could be:
exe:hlint (component)
lib:hlint (component)

So I have to specify that I want to install the executable, not the library? I thought install was only for building executables.

> cabal v2-install hlint:exe:hlint
...
cabal: Path '/home/mitchell/.local/bin/hlint' already exists. Use --overwrite-policy=always to overwrite.

Ok... not a bad default I suppose, but I was unable to figure out the .cabal/config syntax for this option. overwrite-policy = always doesn't work.

> cabal v2-install hlint:exe:hlint --overwrite-policy=always

Works, but takes a couple seconds. Kind of annoying after just running without the last --overwrite-policy=always, it feels like this should not have to do any more work.

[gbaz]

I'm not seeing the HasCallStack error in cabal -- perhaps you're not on the latest version?

As for the second question -- v2 install works on libs too, building them into the store and updating the global env file to include them. However, testing the cabal v2-install in a local directory (which does work with latest cabal) does not give me the helpful "ambiguous target" error but just seems to think there is no target if I don't specify -- so that could be improved.

We could also certainly improve the documentation in cabal-config to make clear where overwrite-policy goes... (testing init of a fresh user-config file it looks like it isn't even inserted commented-out to indicate the right location).

[mitchellwrosen]

I'm not seeing the HasCallStack error in cabal -- perhaps you're not on the latest version?

Oops, I was using some version of cabal-install built from source, between 2.4 and 3.0. So apologies, that's not an issue, though oddly the very latest cabal-install can't build hlint-2.2 for an entirely different reason:

cabal: Could not resolve dependencies:
[__0] trying: hlint-2.2 (user goal)
[__1] next goal: ghc-lib-parser (dependency of hlint)
[__1] rejecting: ghc-lib-parser-8.8.0.20190424 (conflict: requires
NondecreasingIndentation)
[__1] rejecting: ghc-lib-parser-0.20190603, ghc-lib-parser-0.20190523,
ghc-lib-parser-0.20190516, ghc-lib-parser-0.20190423,
ghc-lib-parser-0.20190402 (conflict: hlint => ghc-lib-parser==8.8.0.20190424)
[__1] fail (backjumping, conflict set: ghc-lib-parser, hlint)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: hlint, ghc-lib-parser

(but that's not relevant to this ticket)

[typedrat]

This is a dupe of #5978.

[gbaz]

I think the other half of the issue, which is the overwrite-policy, is fixed by #5942