How to do Rust-like tests in Haskell

rusty-haskell-tests.cabal

cabal-version: 3.0

name: rusty-haskell-tests
version: 0.0.1.0
synopsis: How to do Rust-like tests in Haskell

library
  hs-source-dirs:
    src
  build-depends:
    base
  exposed-modules:
    Simple

test-suite tests
  type: exitcode-stdio-1.0
  hs-source-dirs:
    src
  main-is: Simple.hs
  build-depends:
    base,
    hspec
  cpp-options: -DRUSTY_TESTS

Simple.hs

{-# LANGUAGE CPP #-}

#ifndef RUSTY_TESTS
module Simple where
#endif

import Data.Functor (fmap)
import Data.Int (Int)
import Data.String (String)
import Text.Show (show)
#ifdef RUSTY_TESTS
import Test.Hspec
#endif

simpleMap :: [Int] -> [String]
simpleMap = fmap show

#ifdef RUSTY_TESTS
main :: IO ()
main = hspec $ do
  describe "simple" $ do
    it "does a very simple thing" $ do
      simpleMap [1, 2, 3] `shouldBe` ["1", "2", "3"]

    it "uh, fails when it should" $
      simpleMap [1, 2, 3] `shouldBe` ["3", "2", "1"]
#endif

This puts the test driver (main) directly in the source file.

Instead, you could have tests/test.hs or whatever with main and then Simple.hs would just define an Expectation term, and the #ifndef around the module declaration could be removed.

A Cabal feature that would be nice (but maybe violates some fundamental expectations) is if you could specify flags on build-depends, then you wouldn’t need to include the src modules in the test-suite. (NB: This is for the version where you have a separate test driver.)

flag rusty-tests
  description: whether to compile the library with its test
    code enabled, this is internal and shouldn’t be set externally.
  default: False
  manual: True

library
  hs-source-dirs:
    src
  build-depends:
    base
  exposed-modules:
    Simple
  if flag(rusty-tests)
    cpp-options: -DRUSTY_TESTS

test-suite tests
  type: exitcode-stdio-1.0
  hs-source-dirs:
    tests
  main-is: test.hs
  build-depends:
    base,
    -- HERE’s THE THING THAT YOU CAN’T CURRENTLY DO
    rusty-haskell-tests +rusty-tests,
    hspec