Stack vs cabal config files (docs)

snoyberg · snoyberg · commit af5c97f2aa96 · 2018-01-07T06:55:46.000-08:00
diff --git a/doc/stack_vs_cabal_config.md b/doc/stack_vs_cabal_config.md
@@ -0,0 +1,135 @@
+<div class="hidden-warning"><a href="https://docs.haskellstack.org/"><img src="https://rawgit.com/commercialhaskell/stack/master/doc/img/hidden-warning.svg"></a></div>
+
+# stack vs cabal config files
+
+Due to their apparent overlap, the purpose of the following three files can be
+unclear:
+
+* `stack.yaml`
+* A cabal file, e.g. `my-package.cabal`
+* `package.yaml`
+
+The last two are easy to explain: `package.yaml` is a file format supported by
+[hpack](https://github.com/sol/hpack#readme), which adds some niceties on top
+of cabal like YAML syntax support and automatic generation of `exposed-modules`
+lists. However, it's just a frontend to cabal 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 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
+
+That 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
+<https://www.stackage.org/lts-10.2>, 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:
+
+```yaml
+extra-deps:
+- 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 philosoph 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.
diff --git a/doc/yaml_configuration.md b/doc/yaml_configuration.md
@@ -22,6 +22,10 @@ specific options from `~/.stack/global-project/stack.yaml`. When stack is
 invoked inside a stack project, only options from `<project dir>/stack.yaml` are
 used, and `~/.stack/global-project/stack.yaml` is ignored.
 
+*Note 2:* A common source of confusion is the distinction between configuration
+in a `stack.yaml` file versus a cabal file. If you're trying to understand this
+breakdown, see [stack vs cabal config](stack_vs_cabal_config.md)
+
 ## Project-specific config
 
 Project-specific options are only valid in the `stack.yaml` file local to a
diff --git a/mkdocs.yml b/mkdocs.yml
@@ -19,6 +19,7 @@ pages:
   - User guide: GUIDE.md
   - FAQ: faq.md
   - Configuration (Project and Global): yaml_configuration.md
+  - stack.yaml vs cabal config: stack_vs_cabal_config.md
   - Build command: build_command.md
   - Dependency visualization: dependency_visualization.md
   - Docker integration: docker_integration.md
