ghc --make module hierarchy - haskell

Question

I'm trying to build a standalone program for the first time in Haskell, and am having trouble figuring out how to get ghc --make working with the directory organization of my liking. As the moment I have the following tree:
readme.md
src/
Main.hs
Ciphers/
Affine.hs
Shift.hs
Substitution.hs
Tests/
HCTests.hs
With the following imports:
Main.hs:
module Main where
import Tests.HCTests
import Ciphers.Affine
import Ciphers.Shift
import Ciphers.Substitution
HCTests.hs
module Tests.HCTests (unitTests) where
import Ciphers.Substitution
import Ciphers.Affine
import Ciphers.Shift
Affine.hs
module Affine (
affineEnc,
affineDec,
) where
Shift.hs
module Shift (
shiftEnc,
shiftDec
) where
import Affine
Substitution.hs
module Substitution (
substitutionEnc,
substitutionDec,
) where
Based on this - https://en.wikibooks.org/wiki/Haskell/Standalone_programs - It seems to me the following command should at least properly handle the imports in main, although I'm unclear whether the imports in HCTests will work (it seems to me if I read this properly - Specifying "Up The Tree" Haskell Modules - they should).
The command, which I run in the base directory, is:
ghc -O2 --make -i src -o crypto Main.hs
Fails with error:
target `src' is not a module name or a source file
Edit
I have an additional question. Thanks to Zeta's answer, I've got that sorted, however when I run it, I get the following error:
src/Ciphers/Substitution.hs:5:8:
File name does not match module name:
Saw: `Substitution'
Expected: `Ciphers.Substitution'
So my assumption is that I'd solve this via:
Substitution.hs
module Ciphers.Substitution (
substitutionEnc,
substitutionDec,
) where
My question is how then can I handle that Shift.hs needs to import Affine.hs, while I still want both Ciphers.Shift and Ciphers.Affine?

Answer 1

You may not separate the command line option -i and the search path with whitespace, since -i with following whitespace resets the search path.
Use
ghc -O2 --make -isrc -o crypto Main.hs
or
ghc -O2 --make -i./src -o crypto Main.hs
instead.