Resolving dependencies...
Starting     unbeliever-0.11.0.1
Building     unbeliever-0.11.0.1
Failed to install unbeliever-0.11.0.1
Build log ( /home/builder/.cabal/logs/ghc-8.10.2/unbeliever-0.11.0.1-I2EHpJ4j4spH9JMNVbvXR2.log ):
cabal: Entering directory '/tmp/cabal-tmp-25879/unbeliever-0.11.0.1'
Configuring unbeliever-0.11.0.1...
Building unbeliever-0.11.0.1...
Warning: No documentation was generated as this package does not contain a
library. Perhaps you want to use the --executables, --tests, --benchmarks or
--foreign-libraries flags.
cabal: No executables and no library found. Nothing to do.

cabal: Leaving directory '/tmp/cabal-tmp-25879/unbeliever-0.11.0.1'
cabal: Error: some packages failed to install:
unbeliever-0.11.0.1-I2EHpJ4j4spH9JMNVbvXR2 failed during the final install
step. The exception was:
ExitFailure 1