I am attempting to build GHC from source following this page. When installing dependencies, I had to install happy and alex:
$ cabal install alex happy
Resolving dependencies...
Configuring alex-3.2.1...
Configuring happy-1.19.5...
Building happy-1.19.5...
Building alex-3.2.1...
Installed alex-3.2.1
Installed happy-1.19.5
yet when running configure I get an error message:
$ ./configure
...
checking for happy... no
checking for version of happy...
configure: error: Happy version 1.19.4 or later is required to compile GHC.
I am running on Debian stretch:
$ uname -a
Linux <host> 4.8.0-1-amd64 #1 SMP Debian 4.8.5-1 (2016-10-28) x86_64 GNU/Linux
Can anyone suggest an obvious step I could try to resolve this?