How to define multiple executables / Main modules with stack/hpack - haskell

Question

I use stack and a package.yaml file for hpack to compile my haskell project. It has three executables backed by one library. As one would expect the executables are all defining a Main module:
$ head -n1 app/*
==> app/Foo.hs <==
module Main where
==> app/Bar.hs <==
module Main where
==> app/Baz.hs <==
module Main where
And I use this package.yaml which looks very similar (to me) to the one linked from the official docs for hpack (the third one).
name: myproject
dependencies:
- base
library:
source-dirs: src
executables:
foo:
main: Foo.hs
source-dirs: app
dependencies: myproject
bar:
main: Bar.hs
source-dirs: app
dependencies: myproject
baz:
main: Baz.hs
source-dirs: app
dependencies: myproject
But when I stack build I get the error, that the module name does not match
the file name:
Building all executables for `myproject' once. After a successful build of all of them, only specified executables will be rebuilt.
myproject-0.0.0: build (lib + exe)
Preprocessing library for myproject-0.0.0..
Building library for myproject-0.0.0..
[1 of 2] Compiling Lib ( src/Lib.hs, .stack-work/dist/x86_64-linux-tinfo6/Cabal-2.2.0.1/build/Lib.o )
[2 of 2] Compiling Paths_myproject ( .stack-work/dist/x86_64-linux-tinfo6/Cabal-2.2.0.1/build/autogen/Paths_myproject.hs, .stack-work/dist/x86_64-linux-tinfo6/Cabal-2.2.0.1/build/Paths_myproject.o )
Preprocessing executable 'bar' for myproject-0.0.0..
Building executable 'bar' for myproject-0.0.0..
/home/luc/test/app/Baz.hs:1:8: error:
File name does not match module name:
Saw: ‘Main’
Expected: ‘Baz’
|
1 | module Main where
| ^^^^
-- While building package myproject-0.0.0 using:
/home/luc/.stack/setup-exe-cache/x86_64-linux-tinfo6/Cabal-simple_mPHDZzAJ_2.2.0.1_ghc-8.4.4 --builddir=.stack-work/dist/x86_64-linux-tinfo6/Cabal-2.2.0.1 build lib:myproject exe:bar exe:baz exe:foo --ghc-options " -ddump-hi -ddump-to-file -fdiagnostics-color=always"
Process exited with code: ExitFailure 1
The only difference between my setup and the example that I found was the capital letters of the files in app/. And indeed, if I change them to lower case (in the filesystem and the package.yaml) it all builds correctly.
But why is that and where is that documented?

Answer 1

I think app should not be considered as a convention to be a folder of all final applications. In my projects I always split these to foo/Main.hs and bar/Main.hs for foo and bar targets. Hence
package.yaml should contain
executables:
foo:
main: Main.hs
source-dirs: foo
dependencies:
- myproject
bar:
main: Main.hs
source-dirs: bar
dependencies:
- myproject
And treat myproject just as a library for them