gistfile1.txt

mgsloan@computer:~/fpco/ghc-7.10-sandbox$ hsenv --ghc=../ghc-7.10.1.tar.xz 
Creating Virtual Haskell directory structure
Installing GHC
  Installing GHC from ../ghc-7.10.1.tar.xz
Initializing GHC Package database at /home/mgsloan/fpco/ghc-7.10-sandbox/.hsenv/ghc_pkg_db
Copying necessary packages from original GHC package database
  Using user-wide (~/.cabal/packages) Hackage download cache directory
Installing cabal config at /home/mgsloan/fpco/ghc-7.10-sandbox/.hsenv/cabal/config
Installing activate script
Installing cabal wrapper using /home/mgsloan/fpco/ghc-7.10-sandbox/.hsenv/cabal/config at /home/mgsloan/fpco/ghc-7.10-sandbox/.hsenv/bin/cabal
Skipping 'cabal update' step, Hackage download cache already downloaded
  to ~/.cabal/packages/. You can update it manually with 'cabal update'
  (from inside or outside the virtual environment).

To activate the new environment use 'source .hsenv/bin/activate'
mgsloan@computer:~/fpco/ghc-7.10-sandbox$ source .hsenv/bin/activate 
Activating  Virtual Haskell Environment (at /home/mgsloan/fpco/ghc-7.10-sandbox).

Use regular Haskell tools (ghc, ghci, ghc-pkg, cabal) to manage your Haskell environment.

To exit from this virtual environment, enter command 'deactivate_hsenv'.
[hsenv]mgsloan@computer:~/fpco/ghc-7.10-sandbox$ runhaskell Test.hs 
/home/mgsloan/fpco/ghc-7.10-sandbox/.hsenv/ghc_pkg_db/package.cache: GHC.PackageDb.readPackageDb: inappropriate type (not a ghc-pkg db file, wrong file magic number)

For folks that ended up here by searching orso, I figured out how to fix my hsenv:

rm .hsenv/ghc_pkg_db/package.cache
cp .hsenv/ghc/lib/ghc-7.10.1/package.conf.d/package.cache .hsenv/ghc_pkg_db/package.cache

@mgsloan Thanks! That was really useful!