Main.hs

module Main where

import Submod


main :: IO ()
main = return ()

Submod.hs

module Submod where

val :: Int
val = 42

test.cabal

cabal-version:       2.2
name:                test
version:             0.1.0.0
author:              Tom Smeding
maintainer:          tom.smeding@gmail.com
build-type:          Simple

executable test
  hs-source-dirs:      .
  main-is:             Main.hs
  other-modules:       Submod
  default-language:    Haskell2010
  ghc-options:         -O2 -Wall
  build-depends:       base >=4.13 && <4.14