After cabal install random, I try ghc-mod list but it still doesn't see the package.
cabal --version
cabal-install version 1.22.6.0
using version 1.22.4.0 of the Cabal library
ghc-mod --version
ghc-mod version 5.5.0.0 compiled by GHC 7.10.3
I have now also tried adding random-1.1 to my global stack.yaml (trying to make a simple script here...) and installed via stack install random to no avail.
My file seems to run fine with runhaskell though.
When you type stack install random, and then run stack ghci or install with stack, the import succeeds
> import System.Random --succeeds
If you run ghc outside of stack, it won't necessarily use the same setup.