Why does stack not pass my ghc-options to the compiler? - haskell

Question

In the cabal file I specify the GHC options -Wall and -O2:
name: Test
version: 0.1.0.0
build-type: Simple
cabal-version: >=1.8
executable Test
hs-source-dirs: src
main-is: Test.hs
build-depends: base >=4.8 && <4.10
ghc-options: -Wall -O2
When I compile the program Test.hs:
data Color = Red | Green | Blue
foo :: Color -> Int
foo Red = 0
foo Green = 1
-- foo Blue is intentionally missing!!
I get the error:
Preprocessing executable 'Test' for Test-0.1.0.0...
[1 of 1] Compiling Main ( src/Test.hs, .stack-work/dist/x86_64-linux-nopie/Cabal-1.24.2.0/build/Test/Test-tmp/Main.o )
/home/user/Projekte/HaskellTutorials/Test/src/Test.hs:1:1: error:
The IO action ‘main’ is not defined in module ‘Main’
-- While building package Test-0.1.0.0 using:
/home/user/.stack/setup-exe-cache/x86_64-linux-nopie/Cabal-simple_mPHDZzAJ_1.24.2.0_ghc-8.0.2 --build
dir=.stack-work/dist/x86_64-linux-nopie/Cabal-1.24.2.0 build exe:Test --ghc-options " -ddump-hi -ddump-to-file"
Process exited with code: ExitFailure 1
The error about the missing main action is not the problem.
Look at the text at the end of the second last line:
build exe:Test --ghc-options " -ddump-hi -ddump-to-file"
Why do I not see my GHC options -Wall -O2? (I fear, I did some stupid little mistake ...)
PS:
stack version is: Version 1.5.1, Git revision 600c1f01435a10d127938709556c1682ecfd694e (4861 commits) x86_64 hpack-0.17.1
LTS: 8.17

Answer 1

The options -ddump-hi -ddump-to-file are being passed by Stack to Cabal. Cabal then adds them to the options specified in the .cabal file before passing them along to GHC.
If you run Stack with:
stack -v --cabal-verbose
and search through the output, you'll see that your options are in fact being passed to GHC.
As #epsilonhalbe noted, GHC isn't complaining about the more minor issues when it discovers that main is missing, so if you add:
main = undefined
to the bottom of your program, you'll get the warnings you're expecting.