How to install and import a library in haskell? - haskell

Question

In official tutorial Haskell in 5 steps, an example imports a library:
import Control.Parallel
Then I use ghc to compile:
ghc parallel.hs -o parallel -O2 --make -threaded -rtsopts
just as the tutorial tells me to do. (My filename is parallel.hs) But I got:
parallel.hs:1:1: error:
Could not find module ‘Control.Parallel’
Use -v to see a list of the files searched for.
|
1 | import Control.Parallel
| ^^^^^^^^^^^^^^^^^^^^^^^
Then I stackoverflowed it and find this question. So I:
cabal new-install parallel --lib
And get:
Resolving dependencies...
Up to date
Then I compiled it again, but got the same error.
How to solve this? Did I miss any command line arguments?
Well, after cabal install parallel, the library is imported, the program is compiled successfully, and runs as expected.
I'll refer to documentations of cabal about the difference between install and new-install

Answer 1

As #Lorenzo noted, cabal install works fine. With cabal new-install you should have seen a warning that you forgot --lib since parallel does not include an executable and libraries must be explicitly installed via --lib. The full set of functional commands are:
cd $(mktemp -d)
cabal update && cabal new-install --lib parallel
cat <<EOF > parallel.hs
module P where
import Control.Parallel
EOF
ghc -c parallel.hs
ls parallel.o parallel.hi
Yielding:
% cd $(mktemp -d)
% cabal update && cabal new-install --lib parallel
... snipped output ...
% cat <<EOF > parallel.hs
module P where
import Control.Parallel
EOF
% ghc -c parallel.hs
Loaded package environment from /home/tommd/.ghc/x86_64-linux-8.6.4/environments/default
% ls parallel.o parallel.hi
parallel.hi parallel.o