make "cabal test" work

default.nix

@@ -2,6 +2,11 @@ let
   pkgs = import <nixpkgs> { };
   haskellEnv = pkgs.haskellPackages.developPackage {
     root = ./.;
+    modifier = drv:
+      pkgs.haskell.lib.addBuildTools drv (with pkgs.haskellPackages;
+        [
+          cabal-install
+        ]);
   };
 in haskellEnv.overrideAttrs(o: {
   buildInputs = o.buildInputs ++

souplesse.cabal

@@ -75,3 +75,16 @@ executable souplesse
 
     -- Base language which the package is written in.
     default-language: GHC2021
+
+-- mostly copied from
+-- https://functional.works-hub.com/learn/basic-unit-testing-in-haskell-using-hunit-and-cabal-29e47
+
+test-suite tests
+    type: exitcode-stdio-1.0
+    main-is: UnitTest.hs
+    hs-source-dirs:
+        tests
+    build-depends:
+        base >=4.7 && <5
+      , HUnit
+    default-language: GHC2021

tests/UnitTest.hs

@@ -0,0 +1,15 @@
+module Main where
+-- import BasicSum
+import Test.HUnit
+import qualified System.Exit as Exit
+
+test1 :: Test
+test1 = TestCase (assertEqual "should return 3" 3 (1 + 2))
+
+tests :: Test
+tests = TestList [TestLabel "test1" test1]
+
+main :: IO ()
+main = do
+    result <- runTestTT tests
+    if failures result > 0 then Exit.exitFailure else Exit.exitSuccess