Add `cabal packages`

[angerman]

I would like to propose that cabal learns about packages or new-packages, as a wrapper around ghc-pkg. The motivation here, is that one needs to use only a single command, and not need to know about ghc-pkg.

I'd like to see:

cabal packages [--global|--local]

where

cabal packages --global

would be

ghc-pkg list --package-db=$HOME/.cabal/store/ghc-VERSION/package.db

and

cabal packages --local

would be

ghc-pkg list --package-db=dist-newstyle/packagedb

I'm not sure if no flag should be permitted, and which of both scenarios it would be, or if it should list global AND local.

---

Slightly unrelated, but why do we have package.db for global and packagedb for local?

[23Skidoo]

Probably better to add cabal new-exec so that you could do cabal new-exec ghc-pkg, like you can with sandboxes.

[angerman]

@23Skidoo that would still require me to understand that there is ghc-pkg, and what it does. I'm coming from a "do not make me think" point of view.

Assuming I'm new to haskell. I kind of know there is ghc, and I heard there is cabal and stack, (I might by accident also have heard of tinc and mafia). For whatever reason I chose to use cabal. I still can't make sense of Cabal, cabal-install and cabal. What's all this? But I'll ignore this for now. I've just been told to run cabal init and cabal (new-)build, and maybe cabal (new-)repl.

To me, the leap from cabal (new-)build to cabal (new-)exec ghc-pkg, is rather big, I need to know I want ghc-pkg, what is this? Why do I need this? And then I need to run cabal (new-)exec, why do I need to do this? All I want is a list of all globally or locally installed packages.

And cabal --help would likely not list cabal (new-)exec ghc-pkg...

If this line of thought runs counter to cabal's philosophy so be it. I'm likely not acquainted enough with the ways of the cabal yet.

[23Skidoo]

All I want is a list of all globally or locally installed packages.

There's a cabal list --installed command, you may want to look into adding new-list.

[hvr]

@23Skidoo ...wasn't the recommended way to use ghc-pkg in sandboxes via

cabal sandbox hc-pkg        [FLAGS] [--] COMMAND [--] [ARGS]

...

hc-pkg:
  Similar to `ghc-pkg`, but for the sandbox package database. Can be used to
  list specific/all packages that are installed in the sandbox. For
  subcommands, see the help for ghc-pkg. Affected by the compiler version
  specified by `configure`.

[hvr]

@angerman naming nitpick: I'm not sure cabal packages --global displaying the store packages is properly named; in the new-build paradigm, we have 3 package dbs, there is the global package db (where e.g. base resides; in other contexts such packages are also called "preinstalled"), the store-cache package db (can get rather huge and can easily contain multiple versions/instances of the same package), and then the local/inplace package db.

[23Skidoo]

@hvr Yep, right, because it can use a non-default ghc version.

[gbaz]

is this obseleted by new build or handled by #7500 ?

[fgaz]

or by cabal env list #6481 for GHC environments (since the user is no longer exposed to package dbs)