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How to Build with Stack

This tutorial assumes you’ve already installed the Stack build tool. If you haven’t yet, please start with the get started page and then come back here.

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In this article we’ll be talking about how to build full fledged projects consisting of libraries, executables, test suites, and benchmarks in Haskell with Stack.

Start a new project

You can start a new project with the command:

stack new my-project-name

This will use the default project template. You can also choose from many available templates which you can list with

stack templates

For example, if you wanted to the simple template instead, you could run:

stack new my-project-name simple

You should now have a directory named my-project-name (or whatever string you used), with a stack.yaml file inside of it. A stack.yaml file must be present in the root directory of each project, and provides a number of settings. You can edit that file and view the comments, or view the configuration file documentation.

Convert an existing package

If you have an existing Cabal package, you can use the stack init command inside the package directory to initialize a stack.yaml file. Stack will attempt to determine a package set compatible with the packages requested in your .cabal file.

Project versus package

Note the difference in terminology above. This is important: a cabal package is identified by a single .cabal file, and has 0 or 1 libraries, and 0 or more executables, test suites, and benchmarks.

A Stack project has 1 or more cabal packages, and can build them all at the same time. In likely the majority of cases, your Stack project will have just one cabal package in it. However, multi-package projects can be very common for both open source library collections and for structuring commercial applications. Some open source examples of multi-package projects include Yesod, WAI, and Servant.

Building

The basic command for building your project is

stack build

Very likely, you’ll need to first tell Stack to install the appropriate GHC version for your project. You can do this with:

stack setup

or by using the --install-ghc option to stack build:

stack --install-ghc build

Running executables

Let’s suppose your project defines an executable called my-executable. How do you run it? There are three common ways:

  1. stack exec my-executable will modify your PATH variable to include a number of additional directories, including the internal executable destination, and your build tools (like ghc).

  2. stack exec which my-executable will use the which command to find the full path to your executable, which you could then run, without the additional modifications that stack exec implies. If you want to be clever, you could do something like this from your shell:

    $ $(stack exec which my-executable)
    
  3. The stack install command will copy your executables into a user-specific directory, such as $HOME/.local/bin on POSIX systems. The directory will be printed to your console.

Testing

Testing is also straightforward:

stack test

As it happens, this is just a convenience shortcut for:

stack build --test

The same applies to stack bench (for benchmarking) and stack haddock (for building Haddock documentation). What this means is that you can compose these flags to build the code, build the docs, run tests, and run benchmarks:

stack build --test --bench --haddock

Common flags

  • --file-watch will run build in file-watch mode, where it will wait for changes to files and then automatically rebuild. This can be very convenient to run in a terminal while simulatenously editing in another window.
  • --fast will disable optimizations
  • --pedantic turns on -Wall -Werror for GHC (all warnings on, and warnings treated as errors)

So throwing a few of these together:

stack build --test --file-watch --fast --pedantic

What’s next

This is a small taste of the capabilities of Stack for building projects. You can find much more information in the Stack user guide.