[Haskell-beginners] Import errors from a very simple program

[John Sturdy]

Hi,

I'm right at the start of learning Haskell, and I decided to jump from
reading LYAH to writing a simple real-world program.

It's based very closely on
https://github.com/haskell-hvr/cassava/blob/master/examples/NamedBasedGeneric.hs
(I removed the bit that constructs the test file, as I already have a
file), and I've done "cabal install cassava", first as root, and then when
I first go the error, as myself using the sandbox (which didn't change the
output).  I first tried this with running ghc directly, and then followed
the instructions on https://wiki.haskell.org/How_to_write_a_Haskell_program,
and all these get the same errors:

Main.hs:4:1: error:
    Could not find module ‘Data.Csv’
    Locations searched:
      Data/Csv.hs
      Data/Csv.lhs
      Data/Csv.hsig
      Data/Csv.lhsig
  |
4 | import Data.Csv
  | ^^^^^^^^^^^^^^^

Main.hs:6:1: error:
    Could not find module ‘Data.Vector’
    Perhaps you meant Data.Functor (from base-4.11.1.0)
    Locations searched:
      Data/Vector.hs
      Data/Vector.lhs
      Data/Vector.hsig
      Data/Vector.lhsig
  |
6 | import qualified Data.Vector as V
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I don't understand the connection between the name "cassava" and
"Data/Csv", although looking around in the directories that have been
produced, I see that Data/Csv is within the cassava directories.   I'd like
not only to know how to fix this,  but also a pointer for what I should
look at to understand what is going on.

This is on a Debian system.

thanks,

__John (completely new to Haskell, experienced in C, Lisp, Python and
others, if that helps to pitch the level of answers)
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