I was trying to build haddock using stack so I can generate docs for hoogle.
$ stack haddock --haddock-deps
# fails with
2018-05-23 09:50:28.287257: [debug] Run process within /tmp/stack32381/haskell-src-exts-1.20.2/: /home/tester/.stack/setup-exe-cache/x86_64-linux/Cabal-simple_mPHDZzAJ_2.2.0.1_ghc-8.4.2 --builddir=.stack-work/dist/x86_64-linux/Cabal-2.2.0.1 haddock --html --hoogle --html-location=../$pkg-$version/ --haddock-option=--hyperlinked-source
@(src/Stack/Build/Execute.hs:1133:29)
Progress 1/2