cabal sandbox hell: can not resolve depdencies - haskell

Question

I'm trying to build my package using a sandbox. Without it, it compiles and works like a charm. Once I introduce a sandbox though cabal says it can't resolve dependencies. To my eyes these conflicts do not look like conflicts though.
The project structure is as follows:
application (depends on library-base, library-impl1 and library-impl2)
library
├ library-base
├ library-impl1 (depends on library-base)
└ library-impl2 (depends on library-base)
These are the commands I ran in that order
rm -rf ~/.cabal
rm -rf ~/.ghc
cabal update
rm -f cabal.sandbox.config
rm -rf .cabal-sandbox
cabal sandbox init
cabal sandbox add-source $LIB_HOME/library-base
cabal sandbox add-source $LIB_HOME/library-impl1
cabal sandbox add-source $LIB_HOME/library-impl2
cabal install --only-dependencies --force-reinstalls && cabal build
cabal install --only-dependencies --force-reinstalls produces the following output:
Resolving dependencies...
cabal: Could not resolve dependencies:
next goal: lens (dependency of library-base-0.1.0.0)
rejecting: lens-4.15.4/installed-I5C... (conflict: bifunctors==5.4.2, lens =>
bifunctors==5.4.2/installed-Hs7...)
trying: lens-4.15.4
trying: unordered-containers-0.2.8.0/installed-1tq... (dependency of
lens-4.15.4)
next goal: text (dependency of lens-4.15.4)
rejecting: text-1.2.2.2/installed-3EN... (conflict: binary==0.8.5.1, text =>
binary==0.8.3.0/installed-0.8...)
trying: text-1.2.2.2
next goal: hashable (dependency of lens-4.15.4)
rejecting: hashable-1.2.6.1/installed-2nP... (conflict: text==1.2.2.2,
hashable => text==1.2.2.2/installed-3EN...)
rejecting: hashable-1.2.6.1, hashable-1.2.6.0, hashable-1.2.5.0,
hashable-1.2.4.0, hashable-1.2.3.3, hashable-1.2.3.2, hashable-1.2.3.1,
hashable-1.2.3.0, hashable-1.2.2.0, hashable-1.2.1.0, hashable-1.2.0.10,
hashable-1.2.0.9, hashable-1.2.0.8, hashable-1.2.0.7, hashable-1.2.0.6,
hashable-1.2.0.5, hashable-1.2.0.4, hashable-1.2.0.3, hashable-1.2.0.2,
hashable-1.2.0.1, hashable-1.2.0.0, hashable-1.1.2.5, hashable-1.1.2.4,
hashable-1.1.2.3, hashable-1.1.2.2, hashable-1.1.2.1, hashable-1.1.2.0,
hashable-1.1.1.0, hashable-1.1.0.0, hashable-1.0.1.1, hashable-1.0.1.0,
hashable-1.0.0 (conflict: unordered-containers =>
hashable==1.2.6.1/installed-2nP...)
Dependency tree exhaustively searched.
Does anyone have an idea what is going on here?

Answer 1

First I will give an explanation of what the error message says. Then I will attempt to make some guesses about why you have the error. Then I will propose some next steps.
The error says:
Maybe version 4.15.4 of lens will work.
Maybe the installed version 0.2.8.0 of unordered-containers will satisfy lens' dependency on unordered-containers.
Maybe the installed version 1.2.2.2 of text will satisfy lens' dependency on text.
Crap. You asked for version 0.8.5.1 of binary, but the installed version of text was built against version 0.8.3.0. I have to rebuild text.
Maybe the installed version 1.2.6.1 version of hashable will satisfy lens' dependency on hashable.
Crap. We're rebuilding text, a dependency of hashable. I have to rebuild hashable.
Double crap. We already committed to using the installed version of unordered-containers, which depends on the installed version of hashable, which prevents us from rebuilding hashable.
(...and I tried a bunch of other stuff that didn't work, too, but I'm not telling you what, nya nya nya)
Normally sandboxes ignore local package databases, so this suggests to me that where it is talking about "installed versions" above it is either picking these up from your global package database or from your existing sandbox. You claim that you have just created this sandbox and run this cabal install command immediately. If this is true (is it? or is this a second run of the tool? why do you have --force-reinstalls?), it cannot be picking the installed versions from your sandbox, and therefore must be picking them up from your global package database. Installing things to the global package database is generally considered a Bad Idea, because it's much more difficult to correctly clear out a cluttered/broken global package database. Take it under future advisement.
Now, to make progress, I would try one of these two things:
The only part of the above error explanation that you have control over is the "You asked for version 0.8.5.1 of binary" part. Consider relaxing your version constraints on binary to accept the already-installed version 0.8.3.0.
You can often get significantly better error messages out of cabal by cutting down on its search space; because it only prints a part of the search space in its error, you sometimes need to guide it to the problematic part of the space so that it will show you the real problem in its error. If you're pretty sure the installed versions of unordered-containers et al are a good match for your library, consider
cabal install --only-dependencies --constraint 'unordered-containers installed' --constraint 'binary installed'
and so on for any other packages it mentions trying to rebuild that you don't want it to rebuild. Alternately you can put exact version constraints in the --constraint if you don't want one of the installed versions. Don't include --force-reinstalls, basically ever.