What's imported from cabal repl
If you have only an executable declared in your cabal file, then when you cabal repl it will import all(?) custom modules depended on in your main file, as well as
a Main module which contains your main file.
If you have a library exposed, then your cabal repl will only import your exposed
modules. You also won’t get access to the magically generated Main module after you
drop into ghci. Even if you export both libraries and executables, only the rules
of the library will be imported.
You can sidestep this with: cabal repl <executable name> – however you should also
keep in mind that your named executible may have a colliding name with the name of
the library (found in name at the top of your cabal file).
If you want to import a specific module into your cabal repl, you can do the same trick:
cabal repl <module name>
However you can still wind up with a namespace collision. Since you cabal allows us to import files without needing to specify the full or relative path, you can import a file (containing the module you want) and hope that cabal will fuzzy-match what you are expecting.
So if you don’t titlecase the module name, say if you enter hello, cabal may think
that you are referencing a file and need further disambiguation between the module
name, Hello, and the file that contains it, hello.hs or Hello.hs. You may, of
course, also pass the full or relative path as a means of disambiguation.