build_command.md

Build command

Overview

The primary command you use in stack is build. This page describes the build command's interface. The goal of the interface is to do the right thing for simple input, and allow a lot of flexibility for more complicated goals. See the build command section of the user guide for info beyond the CLI aspects of the build command.

Synonyms

One potential point of confusion is the synonym commands for build. These are provided to match commonly expected command line interfaces, and to make common workflows shorter. The important thing to note is that all of these are just the build command in disguise. Each of these commands are called out as synonyms in the --help output. These commands are:

• stack test is the same as stack build --test
• stack bench is the same as stack build --bench
• stack haddock is the same as stack build --haddock
• stack install is the same as stack build --copy-bins

The advantage of the synonym commands is that they're convenient and short. The advantage of the options is that they compose. For example, stack build --test --copy-bins will build libraries, executables, and test suites, run the test suites, and then copy the executables to your local bin path (more on this below).

Components

Components are a subtle yet important point to how build operates under the surface. Every cabal package is made up of one or more components. It can have 0 or 1 libraries, and then 0 or more of executable, test, and benchmark components. stack allows you to call out a specific component to be built, e.g. stack build mypackage:test:mytests will build the mytests component of the mypackage package. mytests must be a test suite component.

We'll get into the details of the target syntax for how to select components in the next section. In this section, the important point is: whenever you target a test suite or a benchmark, it's built and also run, unless you explicitly disable running via --no-run-tests or --no-run-benchmarks. Case in point: the previous command will in fact build the mytests test suite and run it, even though you haven't used the stack test command or the --test option. (We'll get to what exactly --test does below.)

This gives you a lot of flexibility in choosing what you want stack to do. You can run a single test component from a package, run a test component from one package and a benchmark from another package, etc.

One final note on components: you can only control components for local packages, not dependencies. With dependencies, stack will always build the library (if present) and all executables, and ignore test suites and benchmarks. If you want more control over a package, you must add it to your packages setting in your stack.yaml file.

Target syntax

In addition to a number of options (like the aforementioned --test), stack build takes a list of zero or more targets to be built. There are a number of different syntaxes supported for this list:

• package, e.g. stack build foobar, is the most commonly used target. It will try to find the package in the following locations: local packages, extra dependencies, snapshots, and package index (e.g. Hackage). If it's found in the package index, then the latest version of that package from the index is implicitly added to your extra dependencies.

  This is where the --test and --bench flags come into play. If the package is a local package, then all of the test suite and benchmark components are selected to be built, respectively. In any event, the library and executable components are also selected to be built.

• package identifier, e.g. stack build foobar-1.2.3, is usually used to include specific package versions from the index. If the version selected conflicts with an existing local package or extra dep, then stack fails with an error. Otherwise, this is the same as calling stack build foobar, except instead of using the latest version from the index, the version specified is used.

• component. Instead of referring to an entire package and letting stack decide which components to build, you select individual components from inside a package. This can be done for more fine-grained control over which test suites to run, or to have a faster compilation cycle. There are multiple ways to refer to a specific component (provided for convenience):

  • packagename:comptype:compname is the most explicit. The available comptypes are exe, test, and bench.
    • Side note: When any exe component is specified, all of the package's executable components will be built. This is due to limitations in all currently released versions of Cabal. See issue#1046
  • packagename:compname allows you to leave off the component type, as that will (almost?) always be redundant with the component name. For example, stack build mypackage:mytestsuite.
  • :compname is a useful shortcut, saying "find the component in all of the local packages." This will result in an error if multiple packages have a component with the same name. To continue the above example, stack build :mytestsuite.

• directory, e.g. stack build foo/bar, will find all local packages that exist in the given directory hierarchy and then follow the same procedure as passing in package names as mentioned above. There's an important caveat here: if your directory name is parsed as one of the above target types, it will be treated as that. Explicitly starting your target with ./ can be a good way to avoid that, e.g. stack build ./foo

Finally: if you provide no targets (e.g., running stack build), stack will implicitly pass in all of your local packages. If you only want to target packages in the current directory or deeper, you can pass in ., e.g. stack build ..

To get a list of the available targets in your project, use stack ide targets.

Controlling what gets built

Stack will automatically build the necessary dependencies. See the build command section of the user guide for details of how these dependencies get specified.

In addition to specifying targets, you can also control what gets built, or retained, with the following flags:

• --haddock, to build documentation. This may cause a lot of packages to get re-built, so that the documentation links work.

• --force-dirty, to force rebuild of packages even when it doesn't seem necessary based on file dirtiness.

• --reconfigure, to force reconfiguration even when it doesn't seem necessary based on file dirtiness. This is sometimes useful with custom Setup.hs files, in particular when they depend on external data files.

• --dry-run, to build nothing and output information about the build plan.

• --only-dependencies, to skip building the targets.

• --only-snapshot, to only build snapshot dependencies, which are cached and shared with other projects.

• --keep-going, to continue building packages even after some build step fails. The packages which depend upon the failed build won't get built.

• --keep-tmp-files, to keep intermediate files and build directories that would otherwise be considered temporary and deleted. It may be useful to inspect these, if a build fails. By default, they are not kept.

• --skip, to skip building components of a local package. It allows you to skip test suites and benchmark without specifying other components (e.g. stack test --skip long-test-suite will run the tests without the long-test-suite test suite). Be aware that skipping executables won't work the first time the package is built due to an issue in cabal. This option can be specified multiple times to skip multiple components.

Flags

There are a number of other flags accepted by stack build. Instead of listing all of them, please use stack build --help. Some particularly convenient ones worth mentioning here since they compose well with the rest of the build system as described:

• --file-watch will rebuild your project every time a file changes, by default it will take into account all files belonging to the targets you specify, alternatively one could specify --watch-all which will make Stack watch any local files (from project packages or from local dependencies)
• --exec "cmd [args]" will run a command after a successful build

To come back to the composable approach described above, consider this final example (which uses the wai repository):

stack build --file-watch --test --copy-bins --haddock wai-extra :warp warp:doctest --exec 'echo Yay, it worked!'

This command will:

• Start stack up in file watch mode, waiting for files in your project to change. When first starting, and each time a file changes, it will do all of the following.
• Build the wai-extra package and its test suites
• Build the warp executable
• Build the warp package's doctest component (which, as you may guess, is a test site)
• Run all of the wai-extra package's test suite components and the doctest test suite component
• If all of that succeeds:
  • Copy generated executables to the local bin path
  • Run the command echo Yay, it worked!

Build output

Starting with Stack 2.1, output of all packages being built scrolls by in a streaming fashion. The output from each package built will be prefixed by the package name, e.g. mtl> Building .... This will include the output from dependencies being built, not just targets.

To disable this behaviour, you can pass --no-interleaved-output, or add interleaved-output: false to your stack.yaml file. When disabled:

• When building a single target package (e.g., stack build in a project with only one package, or stack build package-name in a multi-package project), the build output from GHC will be hidden for building all dependencies, and will be displayed for the one target package.

• By default, when building multiple target packages, the output from these will end up in a log file instead of on the console unless it contains errors or warnings, to avoid problems of interleaved output and decrease console noise. If you would like to see this content instead, you can use the --dump-logs command line option, or add dump-logs: all to your stack.yaml file.