# Profiling with Stack
Configure profiling for a project via
stack. First build the project with the
stack build --profile
GHC flags are not required in the cabal file for this to work (like
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
# 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.
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"
# 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
# Viewing dependencies
To find out what packages your project directly depends on, you can simply use this command:
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
# Installing Stack
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.
# Stack install
By running the command
Stack will copy a executable file to the folder
# 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 there is a segment that looks like:
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
build-depends: base >= 4.7 && < 5
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.:
With the next
stack build Stack will use the LTS 6.9 version and hence download some new dependencies.