Cabal fails to link Paths module to executables in separate directories

[hanshoglund]

In a library+executable package, executables that do not share their source directories with the library sources should be able to refer to exports from the library (as per #81). However the following setup fails to link.

(This repository provides a full project demonstrating the issue, see below for a summary).

Cabal file

library
    build-depends: base >= 4 && < 5
    hs-source-dirs: src
    default-language: Haskell2010
    exposed-modules: FooBar

executable "test-foo-bar"
    build-depends: foo-bar, base >= 4 && < 5
    hs-source-dirs: tools
    default-language: Haskell2010
    main-is: test-foo-bar.hs

Sources

-- src/FooBar.hs
module FooBar where
import Paths_foo_bar (version)
import Data.Version
versionString = showVersion version

-- tools/test-foo-bar.hs
module Main where
import FooBar (versionString)
main = putStrLn $ "The version is " ++ versionString

Output

$ cabal build
[1 of 1] Compiling Main             ( tools/test-foo-bar.hs, dist/build/test-foo-bar/test-foo-bar-tmp/Main.o )
Linking dist/build/test-foo-bar/test-foo-bar ...
Undefined symbols for architecture x86_64:
  "_foozmbarzm0zi2_Pathszufoozubar_version1_closure", referenced from:
      _foozmbarzm0zi2_FooBar_versionString_info in libHSfoo-bar-0.2.a(FooBar.o)
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

[ingemaradahl]

You need to make sure to expose the generated Paths_ module. Add

other-modules:      Paths_foo_bar

to your library section in foo-bar.cabal, and it should link fine. Not a very nice message from GHC though :/