Stack, Cabal etc

Stack is a (not the only) Haskell standard for scaffolding, building, packaging and managing dependencies for a project. It uses Cabal under the covers. The interplay between Stack, Cabal and HPack makes this area a bit of a minefield.

Start a new stack project

In parent directory, where simple is the template from stack templates list:

stack new [project-name] simple

Run ghci

GHCi session in the context of the current project:

stack ghci

Build and run executable

stack build && stack exec package-name-exe

Install a dependency

• For a global package install, (like npm install -g package) use cabal NOT stack:

cabal update && cabal install package

• For a local package install restricted to only a stack project (like npm install --save package):

    dependencies:
    - haskell-hspec
    - hspec
    - QuickCheck

• This will in turn generate an edit to the project's Cabal file project.cabal:

  build-depends:       base
                     , haskell-hspec
                     , hspec
                     , QuickCheck

List depedencies

stack list-dependencies

Pass and argument to stack ghc

stack ghc -- --supported-extensions

Environment setup

What is my global ghc, why, and how do I change it?

Global ghc could have been installed in all sorts of ways, it is found by running:

ghc -v

But really you probably want to be using a Stack managed global setup e.g. outside of a project folder, run one of the following commands:

stack ghci -v
stack ghc -v

This should tell you in the logs that you are using e.g. .stack/global-project/stack.yaml. The resolver field in this file will determine which ghc version you are using.

What is my project GHC, why, and where is it stored?

The project ghc is set via the local stack.yaml, again using the resolver to determine the version. ghc itself will be installed somewhere centrally by Stack.

What are my globally installed packages, why, and where are they stored?

The following command when run globally tells you all of the packages which are installed:

ghc-pkg list

What are my project installed packages, why, and where are they stored?

stack list-dependencies

Where does stack build my executables?

stack path --local-install-root

Useful resources

• Problems with Cabal and how to avoid
• Cabal github repo
• Why is Stack not Cabal
• Stackage is Stable Hackage and packages here have been tested to avoid dependency conflicts
• Stack.yaml versus .Cabal
• Stack FAQs

Notes

The logic for resolving dependencies without and with Stack is as follows:

• Packages GHC can use > Are registered with "ghc-pkg register" > And (almost always) built with Cabal > With build dependencies resolved by cabal-install > From Hackage.

• Packages GHC can use > Are registered with "ghc-pkg register" > And (almost always) built with Cabal > With build dependencies resolved by stack > From Stackage (if possible...) or Hackage

Stackage specifies a 'resolver'

a GHC version, a number of packages available for installation, and various settings like build flags"

Global Stack managed dependencies:

• /etc/stack/config.yaml - for system global non-project default options

• ~/.stack/config.yaml - for user non-project default options

When stack is invoked outside a stack project it will source project 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.