`cabal init` should not use same hs-source-dirs when choosing "Library and Executable"

[phadej]

This is bad practice as if an exe component depends on a lib component, GHC will recompile the modules, and not use the compiled library: double compilation of same modules.

---

An alternative is to implement a "don't compile modules we didn't told you to" to GHC, but I think this small step is still a good engineering practice, even cabal could tell (future) GHC to be less smart.

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See #6143 (comment)

[kindaro]

Actually I think this was (almost) fixed in #5740 by introducing --application-dir. This is
how it looks in the most recent cabal-install version:

% cabal init
...
Application (Main.hs) directory:
 * 1) (none)
   2) src-exe
   3) app
   4) Other (specify)
Your choice? [default: (none)] 
Library source directory:
 * 1) (none)
   2) src
   3) lib
   4) src-lib
   5) Other (specify)
Your choice? [default: (none)] 
...

So, if the person running the program so desires, they could specify a distinct directory for the
executable code such as app. The default is none, which is the project root directory.
(Though the test suite does get placed to a subdirectory by default.) If all defaults are
accepted, the file hierarchy looks like this:

├── x.cabal
├── test/
│   └── MyLibTest.hs
├── Setup.hs
├── MyLib.hs
├── Main.hs
├── LICENSE
└── CHANGELOG.md

So it seems to me the only thing we may be doing here is re-set the default to app for the
executable and to lib for the library. Should we?

[m-renaud]

Also, the most recent version of cabal-install allows you to configure this in your ~/.cabal/config file. There is a new init subsection where you can set the default source-dir and application-dir, among others:

init
  ...
  -- application-dir:
  -- source-dir:

[m-renaud]

Circling back to this, given that this is configurable we should be able to avoid too much bikeshedding on whether we use lib or src for source-dir since folks can set the default to whatever they'd prefer.

I'll put together a PR changing the defaults for source-dir to lib/ and application-dir to app/.