How to use HUnit in Stack - haskell

Question

I have a stackproject stapro with a file app/Main.hs
module Main where
import Lib
main = putStrLn "This is main"
foo::Int ->Int
foo = (+1)
and a file test/Spec.hs
module Spec where
import Test.HUnit
import Main (foo)
main :: IO ()
main = putStrLn "Test suite not yet implemented"
testFoo :: Test
testFoo = TestCase $ assertEqual "Should return 2" 2 (foo 1)
When I try to execute the tests however
$ stack test
While constructing the BuildPlan the following exceptions were encountered:
-- While attempting to add dependency,
Could not find package Main in known packages
-- Failure when adding dependencies:
Main: needed (-any), stack configuration has no specified version
needed for package stapro-0.1.0.0
My .cabal file is
name: stapro
version: 0.1.0.0
...
build-type: Simple
-- extra-source-files:
cabal-version: >=1.10
library
hs-source-dirs: src
exposed-modules: Lib
build-depends: base >= 4.7 && < 5
default-language: Haskell2010
executable stapro-exe
hs-source-dirs: app
main-is: Main.hs
ghc-options: -threaded -rtsopts -with-rtsopts=-N
build-depends: base
, stapro
default-language: Haskell2010
test-suite stapro-test
type: exitcode-stdio-1.0
hs-source-dirs: test
main-is: Spec.hs
build-depends: base
, stapro
, HUnit
, Main
ghc-options: -threaded -rtsopts -with-rtsopts=-N
default-language: Haskell2010
...

Answer 1

It looks like you're trying to depend on the executable (Main in build-depends of the test-suite section), so that you can test it in your test suite. That doesn't work, in fact you can't really test your executable at all.
Remove Main from the build-depends. Move all the code you want to test into your library.