Hello all, I am facing a problem that I assume others have seen before, but am stumped for a solution. It's not about Haskell per se, but about how to organize my haskell code to play nicely with the build tools. #The Setup I am currently developing a bitcoin trading agent. I produce 2 executables: the "Trader" and the "Tax" calculator. These 2 executables share a common dependency to talk to a SQLite3 database in their "Persistence" layer. (I actually have 3 levels of dependency, but we'll get to that later) I spend most of my time working on the Trader and the Persistence layer. These are just different source code files: Trader.hs and Persistence.hs. The problem is that as I update the Persistence.hs file I break Tax.hs. Because I am "lazy" and only really have to calculate taxes once I year. I do *not* want to have to continuously modify the Tax.hs file to keep up with the development in the Persistence layer. I just want it to use an old version, so that I only have to bring it up to speed later. In other words, I want to: 1. always be able to compile both executables, 2. delay updating an executable because of changes in a common dependency as much as possible. I want to be lazy and delay updating my Tax.hs code to work with latest version of the Persistence layer a much as possible. At the same time, the Trader and Persistence layer are worked on simultaneously. #My Current Solution The immediate solution that comes to mind is to put these files in 3 different packages. That way, I can use version 1.5 of the Persistence layer for the Tax calculator, but still develop version 2.0 at the same time as working on the Trader. I can use 'cabal sandbox add-source' to immediately get changes made to the Persistence layer as I develop the Trader. There is one minor problem with this approach: I have to manually track the version numbers. I need to put the proper version number in the "build-depends" section in the .cabal file. For example, for Tax.cabal I would have: "build-depends: persistence == 1.5". There is a different approach that is completely orthogonal to using Cabal packages. I can put each of the 3 files (Trader.hs, Tax.hs and Persistence.hs) in 3 different repositories and use git submodules. The Persistence layer would be a submodule of both the Trader and the Tax repos. As each submodule just points to a specific commit in the Persistence repo, I don't have to manually track version numbers and the Trader and Tax calculator can point to different commits and use different versions of the Persistence layer. The git solution is really convenient because when you clone a repo (with --recursive), it comes with the correct versions of the dependencies you need. This is what I did. #The Problem The problem I am facing is that I now have 3 (or more) levels and multiple dependencies. There are 2 changes to the original setup: 1. Persistence.hs now imports MarketModel.hs and that also changes pretty quickly as I develop the Trader. 2. The Trader now also depends on ProfitCalculator.hs that also imports MarketModel.hs Using the 'cabal sandbox add-source' solution now requires that I "add-source" the MarketModel package too. Otherwise, any change I make to this package will not be tracked. In fact, I have to "add-source" two different versions of MarketModel (in general, there's an exponential blow up as the number of levels increases). The manual tracking of version numbers in the cabal file also becomes critical. If I forget to update a version number, the package will be reinstalled over an existing one that's being used. This quickly becomes a mess. The git submodule solutions doesn't work either. Although GHC does the right thing and only complains about two modules with the same name if I use a symbol that is ambiguous. Cabal sets the paths for "import"ed modules in the "hs-source-dirs:" and that is global to the whole package. I can't say: 1. first, compile the Persistence.hs file by "import"ing files in the path /trader/persistence/marketmodel, 2. now, compile the ProfitCalculator.hs by "import"ing files in the path /trader/profitcalculator/marketmodel. That path is the same for the whole project and I can't use different versions of MarketModel.hs In short, it seems no solution works anymore. Is there a way to make this work? Is there another way for me to be lazy about updating my dependencies when building multiple targets? Any pointers on this would be much appreciated! Thanks, Dimitri