# How to create a simple package in Julia

Shuvomoy Das Gupta

March 22, 2020

Thomson's Rule for First-Time Telescope Makers: "It is faster to make a four-inch mirror then a six-inch mirror than to make a six-inch mirror."

In this blog, we discuss how to create a simple package in Julia that computes the proximal operator of a convex quadratic function. The goal is just to illustrate how to create a structured solver package, the code underlying the solver is very inefficient. Let us call our package TestPackage.

We will go through the following 6 steps in order to create a fully functioning package in Julia:

• Step 0. The inner mechanism of the package

• Step 1. Ensure Git and Github are set up properly

• Step 2. Install a few helpful packages for package development

• Step 3. Create the blank package TestPackage using PkgTemplates

• Step 5. Test the package and upload the files to Github

Now let us take a look at the steps in detail.

### Step 0. The inner mechanism of the package

In this step, we take a look at what our package is intended to do. We are given a quadratic function $f(x) = \frac{1}{2}x^{T}Ax+b^{T}x+c$, where $A \in \mathbf{R}^{n \times n}$ is a positive semidefinite matrix, $b\in\mathbf{R}^{n}$ is a column vector, and $c\in\mathbf{R}$ is a scalar. Our goal is to compute the proximal map of the function $f$ at a point $v\in\mathbf{R}^{n}$ for the parameter $\gamma\in\mathbf{R}_{++}$, which is given by the following formula:

$\mathbf{prox}_{\gamma f}(v)=(I+\gamma A)^{-1}(v-\gamma b).$

#### Structure of the package

We can structure our package in two parts: source code in the src folder, and a bunch of test files in the test folder. We will use a julia package named PkgTemplates that will create the aforementioned structure automatically for us. For now, let us just briefly take a look at the skeleton of our code.

1. src folder: The src folder has the following parts.

• Types.jl In this julia file, we store different data types to represent our problem data, parameter, and the output. The problem data essentially comprises of $A,b,c$. We will construct a data type testPackageProblem that will contain these matrices. Then, in the proximal map computation we have the parameter $\gamma$, which can be seen as a setting for our solver; we will create a data type called testPackageSetting that will contain this. Finally, our output vector will be stored in a data type named testPackageResult.

• Utils.jl This file contains a function for matrix inversion using the dependecy package LinearAlgebra.

• TestPackage.jl This is the main julia file corresponding to our package, indicated by its name. It will include both Types.jl and Utils.jl, and contain the main solver function called solver_prox_quad.

2. test folder: The test folder has file called runtests.jl, where we run our solver to different test datasets and see if everything is working as expected.

### Step 1. Ensure Git and Github are set up properly

Our package is going to reside in Github for version control purpose, so we need to create a free account first. Also, for version control, we need to have git installed on our computer as well. A very good instruction on how to perform these steps can be found here.

### Step 2. Install a few helpful packages for package development

Two very nice packages that can facilitate our package development by a significant margin are PkgTemplates and Revise. We install both packages first from Julia REPL.

using Pkg
Pkg.add("PkgTemplates")
Pkg.add("Revise")

A very convenient thing to do is to load Revise during startup. For this, please follow the instructions at this link.

### Step 3. Create the blank package TestPackage using PkgTemplates

using PkgTemplates

Now create a specific template for our project.

template = Template(user = "shuvomoy") # change it to your specification

Now let us generate the package (empty for now).

generate("TestPackage", template)

As seen in the output above, the package is located at the folder C:\Users\shuvo\.julia\dev\TestPackage, copy this location; we are going to use it soon.

Adding the project to the Julia Package Manager. We need to go through the following steps, so that Julia's package manager knows about our project.

First, in the Julia REPL type the following:

cd("C:\\Users\\shuvo\\.julia\\dev\\TestPackage")
# alternatively The following will also work
# cd(joinpath(DEPOT_PATH, "dev", "TestPackage"))
] activate # this will get us into the main Julia environment
] dev . # this adds our package TestPackage
] st # this is to see the change in our default package list

### Step 4. Create the julia files

#### Files in src folder ##

Description of Utils.jl

Create a file named Utils.jl in the src folder and copy the following code into it.

# create a file named Utils.jl and copy the following code
import LinearAlgebra # note that we are using another package LinearAlgebra in our file,
# so we need to add this package to the list of dependencies for our package

function mat_inv(A)
return LinearAlgebra.pinv(A)
end

Note that in the code above, we have used the package LinearAlgebra. So, we need to add this package to the list of dependencies for our package. We can do that as follows.

] activate TestPackage
] add LinearAlgebra

To quit the active environment and return to the base again we can run the following.

] activate

Description of Types.jl

Create a file named Types.jl in the src folder and copy the following code into it.

# testPackageResult represents the result
struct testPackageResult
x::Array{Float64}
end

# testPackageProblem contains the problem data

mutable struct testPackageProblem
A::Array{Float64,2}
b::Array{Float64}
c::Float64
# constructor
function testPackageProblem(A,b,c)
new(A,b,c)
end
end

# testPackageSetting contains the problem setting, i.e., the parameter γ as set by the user

struct testPackageSetting
γ::Float64
# constructor
function testPackageSetting(;γ = 1)
new(γ)
end
end

Description of TestPackage.jl

Create TestPackage.jl file in the src folder, and copy the following code in the file. Note that we are including both Types.jl and Utils.jl and exporting the name of contents, so that when we invoke using TestPackage, we can use those functions.

module TestPackage

include("./Types.jl")

export testPackageResult, testPackageProblem, testPackageSetting # so that the user can uses the types from the main module, see how to use this types in runtest.jl

include("./Utils.jl")

export mat_inv # so that the we can use the functions from the main module

function solver_prox_quad(v, problem::testPackageProblem, setting::testPackageSetting)
# extract the problem data
A = problem.A
b = problem.b
c = problem.c
# extract the problem settings
γ = setting.γ
# compute the proximal parameter
mat_prox_quad = LinearAlgebra.pinv(LinearAlgebra.I + γ*A)
end

end # module

#### Files in test folder

Now that we have created the basic files for our package in the src folder, it is time now to test our code. To that goal, we create a file named runtests.jl in the test folder, where we have the following code.

#### Files in test folder ##
using TestPackage
using Test

@testset "testing for a certain set of data" begin
# Write your own tests here.
A = [ 2.67485     0.186124   0.583946  -0.145416   0.401269;
-0.17813     0.526082  -0.325439  -1.46627   -3.09161 ;
-1.52653    -0.631375  -0.618636  -2.36654    0.291429;
-0.609349   -1.0533    -0.323403   1.34789   -0.409167;
0.0687345  -0.946264  -0.118661  -1.49908   -1.30312]

A = A*A'

A = (A+A')/2

b = [ 0.38052039048801956 ;
0.5554436732038299 ;
1.7334160203093987 ;
0.4470254916964832 ;
1.7676230141050555]

c = 1.0

v = ones(5)

problem_data = testPackageProblem(A, b, c)

problem_setting = testPackageSetting(γ = 2.0)

result_approx = [0.10055689253917299, 0.24322623669390434, 0.0693976149433379, 0.09597235131516035, -0.530323239453701]

@test solver_prox_quad(v, problem_data, problem_setting) ≈ result_approx

end

# we can potentially create more matrices like that

### Step 5. Test the package and upload the files to Github

Now we are in a position to test our package. We can do that in one of the following ways. We can just run the runtests.jl file from an editor such as Atom, and run it by pressing shift-enter.

Another way is open the Julia REPL, and run the following code.

] test TestPackage

So, everything is working fine!

Add the package to Github. Now, we just add our package to Git version control system. Go to your Github account, create a new repository with name TestPackage.jl. In the package creation page, please keep the boxes unchecked for the README.md, LICENSE, and .gitignore, because these are already taken care of when we created our package using PkgTemplates. Next, drag and drop the folder from our ~/.julia/dev directory (to be specific, the contents of the folder C:\Users\shuvo\.julia\dev\TestPackage) to Github desktop. The final step is to click the publish branch button to upload our files to Github.

We are done creating a simple working Julia package!