hspec failing to import (private) code dependency despite CPP override - haskell

Question

Let's say I have a src file like so:
{-# LANGUAGE CPP #-}
module Alphabet (
#ifdef TEST
alphabet
#endif
) where
alphabet :: [Char]
alphabet = "abcdefghijklmnopqrstuvwxyz"
a .cabal file like so:
name: Alphabet
version: 0.1.0.0
library
build-depends: base >=4.8 && <4.9, containers >=0.5 && <0.6, split >=0.2 && <0.3
hs-source-dirs: src
Exposed-modules: Alphabet
default-language: Haskell2010
test-suite alphabet-test
ghc-options: -Wall -Werror
cpp-options: -DTEST
default-extensions: OverloadedStrings
type: exitcode-stdio-1.0
main-is: Spec.hs
hs-source-dirs: tests
build-depends: Alphabet, base >= 4.8 && < 4.9, containers >= 0.5 && <0.6, split >= 0.2 && < 0.3, hspec, QuickCheck
default-language: Haskell2010
A master test file like so:
{-# OPTIONS_GHC -F -pgmF hspec-discover #-}
and a test file like:
module AphabetSpec (spec) where
import Test.Hspec
import Alphabet (alphabet)
spec :: Spec
spec = do
describe "Alphabet.alphabet" $ do
it "returns the alphabet" $ do
alphabet `shouldBe` "abcdefghijklmnopqrstuvwxyz"
now running `cabal test`:
cabal test
Preprocessing library Alphabet-0.1.0.0...
In-place registering Alphabet-0.1.0.0...
Preprocessing test suite 'alphabet-test' for Alphabet-0.1.0.0...
[1 of 1] Compiling Main ( tests/AlphabetSpec.hs, dist/build/alphabet-test/alphabet-test-tmp/AlphabetSpec.o )
tests/AlphabetSpec.hs:4:27:
Module ‘Alphabet’ does not export ‘alphabet’
Why is my CPP not working as expected? How can I fix it?

Answer 1

Why is my CPP not working as expected?
Two-step building. Since your tests depend on the library, it gets build first. The library doesn't have any CPP options set, therefore alphabet doesn't get exported.
When the tests get built, your library is already compiled, and alphabet doesn't get exported. It's a separation of concerns.
How can I fix it?
There are several tricks to work with "hidden" (e.g. non-exported) functions. For one, you can put them all into a .Internal module. That way, users that want to use the hidden bits can do so rather easily, but the casual user doesn't have too many tools at hand.
Another way to handle this is to drop the dependency in the test and instead add the src directory to the tests:
hs-source-dirs: tests, src
However, this also means that you have to rebuild the whole library for tests, but it will enable using CPP.
A third option is to not test alphabet, but instead the observable behaviour of exported functions that depend on it. So instead of testing alphabet, you would test filterAlpha:
filterAlpha :: String -> String
filterAlpha = filter (`elem` alphabet)
You have to test filterAlpha anyway. If there are many functions that use alphabet, it is likely that you will have some test that will notice a regression if you accidentally change it.

Answer 2

The problem was a simply syntax error. #ifdef is wrong, #ifndef is right