Multiple versions of happy

Simon Peyton Jones simonpj at
Tue Mar 30 13:16:59 UTC 2021

What's the approved mechanism to install multiple versions of happy/alex etc?  Eg I tried to build ghc-9.0 and got this:

checking for makeinfo... no

checking for python3... /usr/bin/python3

checking for ghc-pkg matching /opt/ghc/bin/ghc... /opt/ghc/bin/ghc-pkg

checking for happy... /home/simonpj/.cabal/bin/happy

checking for version of happy... 1.20.0

configure: error: Happy version 1.19 is required to compile GHC.

I so I have to

  1.  Install happy-1.19 without overwriting the installed happy-1.20
  2.  Tell configure to use happy-1.19
What's the best way to do those two things?
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