Backflip

UPDATE: I have a solution, and the beginnings of a theory as to why that solution works. See Below.

This is a stupid haskell package created to ask the question "why do I need to list transitive dependencies in my .cabal file". Basically, I have a library and an executable that uses that library. I want the executable to just depend on the library, but it seems that it needs to transitively depend on the library's dependencies as well.

When I tried to build the package initially, this was the error I got:

backflip fizbin$ cabal --version
cabal-install version 1.22.0.0
using version 1.22.0.0 of the Cabal library
backflip fizbin$ cabal configure
Resolving dependencies...
Configuring backflip-0.1.0.0...
backflip fizbin$ cabal build
Building backflip-0.1.0.0...
Preprocessing library backflip-0.1.0.0...
[1 of 1] Compiling Fizbin.Test.Backflip ( src/Fizbin/Test/Backflip.hs, dist/build/Fizbin/Test/Backflip.o )
[1 of 1] Compiling Fizbin.Test.Backflip ( src/Fizbin/Test/Backflip.hs, dist/build/Fizbin/Test/Backflip.p_o )
In-place registering backflip-0.1.0.0...
Preprocessing executable 'BackflipDemo' for backflip-0.1.0.0...

src/Fizbin/Test/Backflip.hs:13:8:
    Could not find module ‘Data.Array.MArray’
    It is a member of the hidden package ‘array-0.5.0.0’.
    Perhaps you need to add ‘array’ to the build-depends in your .cabal file.
    Use -v to see a list of the files searched for.

src/Fizbin/Test/Backflip.hs:14:8:
    Could not find module ‘Data.Array.ST’
    It is a member of the hidden package ‘array-0.5.0.0’.
    Perhaps you need to add ‘array’ to the build-depends in your .cabal file.
    Use -v to see a list of the files searched for.

What's going on here? Why can't cabal use the library version of backflip, since the demo program itself doesn't depend on array directly?

Solution

The solution, it appears, is to not put BackflipDemo.hs in the same directory as Backflip.hs. The reason is that if you do, when cabal asks ghc to go compile BackflipDemo.hs, ghc will read it, see the import of Fizbin.Test.Backflip and determine "hey, I can see that source code right over here" and will attempt to compile the library a second time, rather than attempting to link in the already compiled version.

Therefore, I now have my main and my library code separated. I'll need to think about how to apply this to the production situation this question comes from.

The state of the repository currently reflects the "solved" variant. To see what I started with, check out the repository at an earlier commit.