5. Nix-style Local Builds

Nix-style local builds are a new build system implementation inspired by Nix. The Nix-style local build system is commonly called “v2-build” for short after the cabal v2-* family of commands that control it. However, those names are only temporary until Nix-style local builds become the default. This is expected to happen soon. For those who do not wish to use the new functionality, the classic project style will not be removed immediately, but these legacy commands will require the usage of the v1- prefix as of Cabal 3.0 and will be removed in a future release. For a future-proof way to use these commands in a script or tutorial that anticipates the possibility of another UI paradigm being devised in the future, there are also v2- prefixed versions that will reference the same functionality until such a point as it is completely removed from Cabal.

Nix-style local builds combine the best of non-sandboxed and sandboxed Cabal:

  1. Like sandboxed Cabal previously, we build sets of independent local packages deterministically and independent of any global state. v2-build will never tell you that it can’t build your package because it would result in a “dangerous reinstall.” Given a particular state of the Hackage index, your build is completely reproducible. For example, you no longer need to compile packages with profiling ahead of time; just request profiling and v2-build will rebuild all its dependencies with profiling automatically.

  2. Like non-sandboxed Cabal today, builds of external packages are cached in ~/.cabal/store, so that a package can be built once, and then reused anywhere else it is also used. No need to continually rebuild dependencies whenever you make a new sandbox: dependencies which can be shared, are shared.

Nix-style local builds were first released as beta in cabal-install 1.24. They currently work with all versions of GHC supported by that release: GHC 7.0 and later.

Some features described in this manual are not implemented. If you need them, please give us a shout and we’ll prioritize accordingly.