Stack

Installing Stack

Mac OSX

brew install haskell-stack

Creating a simple project

To create a project called helloworld run:

stack new helloworld simple

This will create a directory called helloworld with the files necessary for a Stack project.

Structure

File structure

A simple project has the following files included in it:

➜  helloworld ls 
LICENSE          Setup.hs         helloworld.cabal src              stack.yaml

In the folder src there is a file named Main.hs. This is the “starting point” of the helloworld project. By default Main.hs contains a simple “Hello, World!” program.

Main.hs

module Main where

main :: IO ()
main = do
  putStrLn "hello world"

Running the program

Make sure you are in the directory helloworld and run:

stack build # Compile the program
stack exec helloworld # Run the program
# prints "hello world"

Stackage Packages and changing the LTS (resolver) version

Stackage is a repository for Haskell packages. We can add these packages to a stack project.

Adding lens to a project.

In a stack project, there is a file called stack.yaml. In stack.yaml there is a segment that looks like:

resolver: lts-6.8

Stackage keeps a list of packages for every revision of lts. In our case we want the list of packages for lts-6.8 To find these packages visit:

https://www.stackage.org/lts-6.8 # if a different version is used, change 6.8 to the correct resolver number.

Looking through the packages, there is a Lens-4.13.

We can now add the language package by modifying the section of helloworld.cabal:

  build-depends: base >= 4.7 && < 5

to:

  build-depends: base >= 4.7 && 5,
                 lens == 4.13

Obviously, if we want to change a newer LTS (after it’s released), we just change the resolver number, eg.:

resolver: lts-6.9

With the next stack build Stack will use the LTS 6.9 version and hence download some new dependencies.

Build and Run a Stack Project

In this example our project name is “helloworld” which was created with stack new helloworld simple

First we have to build the project with stack build and then we can run it with

stack exec helloworld-exe

Stack install

By running the command

stack install

Stack will copy a executable file to the folder

/Users/<yourusername>/.local/bin/

Profiling with Stack

Configure profiling for a project via stack. First build the project with the --profile flag:

stack build --profile

GHC flags are not required in the cabal file for this to work (like -prof). stack will automatically turn on profiling for both the library and executables in the project. The next time an executable runs in the project, the usual +RTS flags can be used:

stack exec -- my-bin +RTS -p

Viewing dependencies

To find out what packages your project directly depends on, you can simply use this command:

stack list-dependencies

This way you can find out what version of your dependencies where actually pulled down by stack.

Haskell projects frequently find themselves pulling in a lot of libraries indirectly, and sometimes these external dependencies cause problems that you need to track down. If you find yourself with a rogue external dependency that you’d like to identify, you can grep through the entire dependency graph and identify which of your dependencies is ultimately pulling in the undesired package:

stack dot --external | grep template-haskell

stack dot prints out a dependency graph in text form that can be searched. It can also be viewed:

stack dot --external | dot -Tpng -o my-project.png

You can also set the depth of the dependency graph if you want:

stack dot --external --depth 3 | dot -Tpng -o my-project.png