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

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