When installing hindent in a sandbox, install happy first.

[bitemyapp]

cabal install happy in the sandbox was required to make haskell-src-exts happy.

I had a copy of happy at /usr/bin/happy but that didn't satisfy it.

Failed to install haskell-src-exts-1.16.0.1
Build log ( /home/callen/Work/hindent-4.0/.cabal-sandbox/logs/haskell-src-exts-1.16.0.1.log ):
Configuring haskell-src-exts-1.16.0.1...
setup-Simple-Cabal-1.20.0.2-x86_64-linux-ghc-7.8.3: The program 'happy'
version >=1.17 is required but the version of /usr/bin/happy could not be
determined.
cabal: Error: some packages failed to install:
haskell-src-exts-1.16.0.1 failed during the configure step. The exception was:
ExitFailure 1
hindent-4.0 depends on haskell-src-exts-1.16.0.1 which failed to install.

[pharpend]

This is a long-standing problem with cabal's build-tools field.

• Couldn't cabal install without happy and alex jgm/pandoc#1337
• cabal-install doesn't consider build-tools as dependencies haskell/cabal#220