Suggest --lib in cabal v2-install error message

[blamario]

As a newby user trying to experiment through ghci with, say, the lens library, the first step would be to install lens:

$ cabal install lens
Resolving dependencies...
cabal: Cannot build the executables in the package lens because it does not
contain any executables. Check the .cabal file for the package and make sure
that it properly declares the components that you expect.

After this failure, the next step would be to look at cabal help install, and then probably to give up. The error message should prominently suggest --lib as the workaround.

[fgaz]

It DID do that, I implemented it myself… I wonder why it isn't showing.

Also pinging @typedrat, who might have changed some of the logic

[typedrat]

I don’t recall making any changes that would effect it. Do you know what commit added the message?

[fgaz]

f6aaeee (what a nice hash)

[fgaz]

That code is currently in the warnIfNoExes function, which gets called in installExes. Maybe it filters targets the wrong way now? I don't have much time to test it atm

[selinger]

Please fix this as soon as possible, as the --lib option does not even appear in the cabal 3.0.0.0 (and later) documentation, e.g. here: https://www.haskell.org/cabal/users-guide/installing-packages.html#building-and-installing-packages

Is there some documentation that is actually up-to-date, and if yes, where? Thanks!

[rasmussehlin]

I second fixing this. Had to find #6246 to know that "--lib" needed to be added.

[fgaz]

If someone here has the time, a bisect would probably be helpful

[sgraf812]

The warning from f6aaeee doesn't appear in 3.0.0.0 because it landed after 3.0.0.0 was cut, which is the cabal-install that came with GHC 8.10.1 for me.

Edit: Oh, it appears I was wrong and also confused commit dates when I double-checked.

This is the code that landed in 3.0.0.0:

cabal/cabal-install/Distribution/Client/CmdInstall.hs

Lines 648 to 663 in b0e52fa

	warnIfNoExes :: Verbosity -> ProjectBuildContext -> IO ()
	warnIfNoExes verbosity buildCtx =
	when noExes $
	warn verbosity $ "You asked to install executables, "
	<> "but there are no executables in "
	<> plural (listPlural selectors) "target" "targets" <> ": "
	<> intercalate ", " (showTargetSelector <$> selectors) <> ". "
	<> "Perhaps you want to use --lib "
	<> "to install libraries instead."
	where
	targets = concat $ Map.elems $ targetsMap buildCtx
	components = fst <$> targets
	selectors = concatMap snd targets
	noExes = null $ catMaybes $ exeMaybe <$> components
	exeMaybe (ComponentTarget (CExeName exe) _) = Just exe
	exeMaybe _ = Nothing

It appears the difference is in how components are handled. A trace ("exeMaybe:" ++ show exe) (Just exe) is probably enough to identify which component is reponsible for this and finding a fix.

[TakodaS]

As a haskell newbie, I got a step further. Running cabal install --lib packagename gets me the following error:

Resolving dependencies...
cabal.exe: Could not resolve dependencies:
[__0] trying: ghc-8.10.4/installed-8.10.4 (user goal)
[__1] next goal: process (user goal)
[__1] rejecting: process-1.6.13.2, process-1.6.13.1, process-1.6.12.0
(constraint from user target requires ==1.6.11.0)
[__1] rejecting: process-1.6.11.0 (conflict: ghc =>
process==1.6.9.0/installed-1.6.9.0)
[__1] rejecting: process-1.6.10.0, process-1.6.9.0/installed-1.6.9.0,
process-1.6.9.0, process-1.6.8.2, process-1.6.8.1, process-1.6.8.0,
process-1.6.7.0, process-1.6.6.0, process-1.6.5.1, process-1.6.5.0,
process-1.6.4.0, process-1.6.3.0, process-1.6.2.0, process-1.6.1.0,
process-1.6.0.0, process-1.5.0.0, process-1.4.3.0, process-1.4.2.0,
process-1.4.1.0, process-1.4.0.0, process-1.3.0.0, process-1.2.3.0,
process-1.2.2.0, process-1.2.1.0, process-1.2.0.0, process-1.1.0.2,
process-1.1.0.1, process-1.1.0.0, process-1.0.1.5, process-1.0.1.4,
process-1.0.1.3, process-1.0.1.2, process-1.0.1.1, process-1.0.0.0,
process-1.6.13.0 (constraint from user target requires ==1.6.11.0)
[__1] fail (backjumping, conflict set: ghc, process)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: process, ghc

And then I gave up :)

[fgaz]

@TakodaS what were you originally trying to do? install --lib is rarely what you want, especially in beginner situations.

For reference, here's the (code that generates the) new warning that only suggests --lib as the least likely of two options:

cabal/cabal-install/src/Distribution/Client/CmdInstall.hs

Lines 681 to 693 in 7d4ce47

	"@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@\n" <>
	"@ WARNING: Installation might not be completed as desired! @\n" <>
	"@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@\n" <>
	"The command \"cabal install [TARGETS]\" doesn't expose libraries.\n" <>
	"* You might have wanted to add them as dependencies to your package." <>
	" In this case add \"" <>
	intercalate ", " (showTargetSelector <$> selectors) <>
	"\" to the build-depends field(s) of your package's .cabal file.\n" <>
	"* You might have wanted to add them to a GHC environment. In this case" <>
	" use \"cabal install --lib " <>
	unwords (showTargetSelector <$> selectors) <> "\". " <>
	" The \"--lib\" flag is provisional: see" <>
	" https://github.com/haskell/cabal/issues/6481 for more information."

[Mikolaj]

@TakodaS: did you, perchance, run your command while in a directory with a cabal.project file? And what was the package you tried to install?

Regarding the use case from ticket description, there are various alternatives described in #6481 and cabal install --lib seems to be the worst of them (at least in current form). Until we figure out an alternative acceptable to all users, we don't know what alternative to suggest in help messages. Also, quite possibly we won't be overloading cabal install for both exe and lib installation (and both from local packages and from Hackage).

[Mikolaj]

I was just advised, for the original problem in the ticket description, the following incantation is the best: cabal repl --build-depends foo. Given that this is now described at https://cabal.readthedocs.io/en/latest/cabal-commands.html#cabal-v2-repl, could we close this issue? Does it work for everybody? Anything else we could fix? Any objections?

[fgaz]

could we close this issue?

The suggestion still does not appear, does it? that's a regression, and it seems a lot of people are stumbling into it (30 +1s on #6246)

[fgaz]

Nevermind, it does!

> cabal install random
Resolving dependencies...
Up to date
Warning:
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: Installation might not be completed as desired! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
The command "cabal install [TARGETS]" doesn't expose libraries.
* You might have wanted to add them as dependencies to your package. In this
case add "random" to the build-depends field(s) of your package's .cabal file.
* You might have wanted to add them to a GHC environment. In this case use
"cabal install --lib random". The "--lib" flag is provisional: see
https://github.com/haskell/cabal/issues/6481 for more information.

Maybe OP was using an older cabal? I'll close optimistically. If anyone (with recent cabal) still isn't getting that message please leave a comment

[sfogarty]

This issue still emerges when installing a package with no executables.

$ cabal install ansi-terminal
Wrote tarball sdist to
/users/sfogarty/functional-testing/weights-and-gates/dist-newstyle/sdist/weights-and-gates-0.1.0.0.tar.gz
Resolving dependencies...
cabal: Cannot build the executables in the package ansi-terminal because none
of the components are available to build: the executable
'ansi-terminal-example' is marked as 'buildable: False'

$ cabal --version
cabal-install version 3.6.2.0
compiled using version 3.6.2.0 of the Cabal library 

It does not suggest to use cabal install --lib ansi-terminal, which does work to install the package.

[gbaz]

You are using cabal-install 3.6.2.0. The current version of cabal-install is 3.10.1.0