stack.yaml vs cabal package file

Due to their apparent overlap, the purpose of the following three files can be unclear:

  • stack.yaml
  • A cabal package file, e.g. my-package.cabal
  • package.yaml

The last two are easy to explain: package.yaml is a file format supported by hpack. It adds some niceties on top of cabal. For example, hpack has YAML syntax support and will automatically generate of exposed-modules lists. However, it's just a frontend to cabal package files. So for this document, we're instead going to focus on the first two and try to answer:

What's the difference between a stack.yaml file and a cabal package file?

Package versus project

Cabal is a build system, which is used by Stack. Cabal defines the concept of a package. A package has:

  • A name and version
  • 0 or 1 libraries
  • 0 or more executables
  • A cabal file (or, as mentioned above, an hpack package.yaml that generates a cabal file)
  • And a bunch more

The second to last bullet bears repeating: there's a 1-to-1 correspondence between packages and cabal files.

Stack is a build tool that works on top of the Cabal build system, and defines a new concept called a project. A project has:

  • A resolver, which tells it about a snapshot (more on this later)
  • Extra dependencies on top of the snapshot
  • 0 or more local Cabal packages
  • Flag and GHC options configurations
  • And a bunch more Stack configuration

A source of confusion is that, often, you'll have a project that defines exactly one package you're working on, and in that situation it's unclear why, for example, you need to specify an extra depedency in both your stack.yaml and cabal file. To explain, let's take a quick detour to talk about snapshots and how Stack resolves dependencies.

Resolvers and snapshots

Stack follows a rule that says, for any projects, there is precisely 1 version of each package available. Obviously there are many versions of many different packages available in the world. But when resolving a stack.yaml file, Stack requires that you have chosen a specific version for each package available.

The most common means by which this set of packages is defined is via a Stackage Snapshot. For example, if you go to the page, you will see a list of 2,666 packages at specific version numbers. When you then specify resolver: lts-10.2, you're telling Stack to use those package versions in resolving dependencies down to concrete version numbers.

Sometimes a snapshot doesn't have all of the packages you want. Or you want a different version. Or you want to work on a local modification of a package. In all of those cases, you can add more configuration data to your stack.yaml to override the values it received from your resolver setting. At the end of the day, each of your projects will end up with some way of resolving a package name into a concrete version number.

Why specify deps twice?

When you add something like this to your stack.yaml file:

- acme-missiles-0.3

What you're saying to Stack is: if at any point you find that you need to build the acme-missiles package, please use version 0.3. You are not saying "please build acme-missiles now." You are also not saying "my package depends on acme-missiles." You are simply making it available should the need arise.

When you add build-depends: acme-missiles to your cabal file or dependencies: [acme-missiles] to your package.yaml file, you're saying "this package requires that acme-missiles be available." Since acme-missiles doesn't appear in your snapshot, without also modifying your stack.yaml to mention it via extra-deps, Stack will complain about the dependency being unavailable.

You may challenge: but why go through all of that annoyance? Stack knows what package I want, why not just go grab it? The answer is that, if Stack just grabbed acme-missiles for you without it being specified in the stack.yaml somehow, you'd lose reproducibility. How would Stack know which version to use? It may elect to use the newest version, but if a new version is available in the future, will it automatically switch to that?

Stack's baseline philosophy is that build plans are always reproducible*. The purpose of the stack.yaml file is to define an immutable set of packages. No matter when in time you use it, and no matter how many new release happen in the interim, the build plan generated should be the same.

* There's at least one hole in this theory today, which is Hackage revisions. When you specify extra-deps: [acme-missiles-0.3], it doesnt' specify which revision of the cabal file to use, and Stack will just choose the latest. Stack version 1.6 added the ability to specify exact revisions of cabal files, but this isn't enforced as a requirement as it's so different from the way most people work with packages.

And now, how about the other side: why doesn't Stack automatically add acme-missiles to build-depends in your cabal file if you add it as an extra-dep? There are a surprising number reasons actually:

  • The cabal spec doesn't support anything like that
  • There can be multiple packages in a project, and how do we know which package actually needs the dependency?
  • There can be multiple components (libraries, executable, etc) in a package, and how do we know which of those actually needs the dependency?
  • The dependency may only be conditionally needed, based on flags, OS, or architecture. As an extreme example, we wouldn't want a Linux-only package to be force-built on Windows.

While for simple use cases it seems like automatically adding dependencies from the cabal file to the stack.yaml file or vice-versa would be a good thing, it breaks down immediately for any semi-difficult case. Therefore, Stack requires you to add it to both places.

And a final note, in case it wasn't clear. The example I gave above used acme-missiles, which is not in Stackage snapshots. If, however, you want to depend on a package already present in the snapshot you've selected, there's no need to add it explicitly to your stack.yaml file: it's already there implicitly via the resolver setting. This is what you do the majority of the time, such as when you add vector or mtl as a build-depends value.