GHCi cannot find modules of my program - haskell

Question

I'm working on a project and I'm using Cabal for management. I've specified directory of source files, modules, all the stuff. All my files have the same names as their corresponding modules, case is preserved.
I can do:
$ cabal configure
$ cabal build
without problems.
However, imagine I have a module Module in file Module.hs, and file File.hs in the same directory. Now, when I'm trying to load File.hs from Emacs for testing, I get the following:
____Could not find module ‘Module’
It is a member of the hidden package ‘ghc-7.8.3’.
Use -v to see a list of the files searched for.
Failed, modules loaded: none.
Full contents of File.hs:
module File where
import Module
How to make it find files of my project?

Answer 1

You can launch the REPL via Cabal like so:
# cabal repl
This is the same as running ghci, but will take into account any additional dependencies installed by cabal install your local or sandbox package repository.

Answer 2

You need to tell GHCi where to find your source files. For example, if your project directory is ./foo and you have your source files in ./foo/src you need to say (from your project directory):
:set -isrc
at the command prompt in GHCi. You will then have access to private members in your sourc file loaded with C-c C-l.
You also need to make sure that you haven't cabal installed your package, otherwise the package will be loaded, not the project source files.

Answer 3

I had the same problem, fixed it, and decided to write about my troubleshooting. This might help new people learning Haskell. Read on.
I was playing around with this example code.
http://zvon.org/other/haskell/Outputdirectory/getCurrentDirectory_f.html
Code:
import Directory
main = aaa "/tmp/FOO"
aaa ddd = do createDirectory ddd
setCurrentDirectory ddd
d <- getCurrentDirectory
print d
writeFile "aaa" "HELLO"
l <- getDirectoryContents d
print l
I noticed that they are using this package.
https://hackage.haskell.org/package/directory-1.3.6.2/docs/System-Directory.html
So I installed it with this commands:
cabal update
cabal install directory
Compiling the example code with ghc failed with this error message.
Could not find module `Directory'
Use -v to see a list of the files searched for.
|
4 | import Directory
| ^^^^^^^^^^^^^^^^
I was stuck for a while until I changed the import line to this:
import System.Directory
After this change ghc could compile successfully.
Conclussion: are you sure you are importing properly?