Can not load a file that uses standard libraries in haskell

Question

Hi I use GHCI and can normally load my files. Now I need to load a file that uses random. I get this error.

Chatterbot.hs:3:1: error:
    Could not find module ‘         


        

Answer 1

The System.Random module is part of the random package, not of the base package. You thus should install it. If you for example use cabal, you can install it with cabal install random.

If you use Haskell stack for example, you can add it to your package-name.cabal file:

  -- …
  build-depends:
      base >=4.7 && <5
    , random >=1.0 && <2

Answer 2

If you are using a version of cabal-install >= 3.0 (check with cabal --version), then, instead of creating a cabal package, you can move to an empty folder and type:

cabal install --lib --package-env . random

This creates a .ghc.environment file in the folder. ghci sessions started inside that folder will pick it up, and they will be able to import System.Random.

If that works correctly, you might want to install "random" globally so that any invocation of ghci can use it:

cabal install --lib random

This creates/modifies the global environment file located at ~/.ghc/$ARCH-$OS-$GHCVER/environments/default.