Package development

Organization of the code is more important with the increasing size of the project and the number of contributors and users. Moreover, it will become essential when different codebases are expected to be combined and reused.

  • Julia was designed from the beginning to encourage code reuse across different codebases as possible
  • Julia ecosystem lives on a namespace. From then, it builds projects and environments.

Namespaces and modules

Namespace logically separate fragments of source code so that they can be developed independently without affecting each other. If I define a function in one namespace, I will still be able to define another function in a different namespace even though both functions have the same name.

  • prevents confusion when common words are used in different meaning:

    • Too general name of functions "create", "extend", "loss",
    • or data "X", "y" (especially in mathematics, think of π)
    • may not be an issue if used with different types
  • Modules is Julia syntax for a namespace


module MySpace
function test1()
function test2()
export test1

Function include copies content of the file to this location (will be part of the module).

Creates functions:


For easier manipulation, these functions can be "exported" to be exposed to the outer world (another namespace).

Keyword: using exposes the exported functions and structs:

using .MySpace

The dot means that the module was defined in this scope.

Keyword: import imports function with availability to redefine it.


using MySpaceMySpace.test1
using MySpace: test1test1
import MySpaceMySpace.test1*
import MySpace: test1test1*
import MySpace: test2test2*
  • symbol "*" denotes functions that can be redefined
 using  .MySpace: test1
 test1()=println("new test")
 import  .MySpace: test1
 test1()=println("new test")


When importing/using functions with name that is already imported/used from another module:

  • the imported functions/structs are invalidated.
  • both functions has to be accessed by their full names.


  • It may be easier to cherry pick only the functions we need (rather than importing all via using)
  • rename some function using keyword as
    import MySpace2: test1 as t1


Modules can be used or included within other modules:

module A
module B
   module C
       c = 2
   b = C.c # you can read from C    (by reference)
   using ..A: a
   # a= b # but not write to A

REPL of Julia is a module called "Main".

  • modules are not copied, but referenced, i.e. B.b===B.C.c
  • including one module twice (from different packages) is not a problem
  • Julia 1.9 has the ability to change the contextual module in the REPL: REPL.activate(TestPackage)


The fact that Julia can redefine a function in a Module by importing it is used by package Revise.jl to synchronize REPL with a module or file.

So far, we have worked in REPL. If you have a file that is loaded and you want to modify it, you would need to either:

  1. reload the whole file, or
  2. copy the changes to REPL

Revise.jl does the latter automatically.

Example demo:

using Revise.jl

Works with:

  • any package loaded with import or using,
  • script loaded with includet,
  • Base julia itself (with Revise.track(Base))
  • standard libraries (with, e.g., using Unicode; Revise.track(Unicode))

Does not work with variables!

How it works: monitors source code for changes and then does:

for def in setdiff(oldexprs, newexprs)
    # `def` is an expression that defines a method.
    # It was in `oldexprs`, but is no longer present in `newexprs`--delete the method.
    delete_methods_corresponding_to_defexpr(mod, def)
for def in setdiff(newexprs, oldexprs)
    # `def` is an expression for a new or modified method. Instantiate it.
    Core.eval(mod, def)

See Modern Julia Workflows for more hints

Namespaces & scoping

Every module introduces a new global scope.

  • Global scope

    • No variable or function is expected to exist outside of it
    • Every module is equal to a global scope (no single "global" exists)
    • The REPL has a global module called Main
  • Local scope

    Variables in Julia do not need to be explicitly declared, they are created by assignments: x=1.

    In local scope, the compiler checks if variable x does not exist outside. We have seen:


    The rules for local scope determine how to treat assignment of x. If local x exists, it is used, if it does not:

    • in hard scope: new local x is created
    • in soft scope: checks if x exists outside (global)
      • if not: new local x is created
      • if yes: the split is REPL/non-interactive:
        • REPL: global x is used (convenience, as of 1.6)
        • non-interactive: local x is created
  • keyword local and global can be used to specify which variable to use

From documentation:

ConstructScope typeAllowed within
module, baremoduleglobalglobal
structlocal (soft)global
for, while, trylocal (soft)global, local
macrolocal (hard)global
functions, do blocks, let blocks, comprehensions, generatorslocal (hard)global, local


f()= x=3
@show x;
x = 1
for _ = 1:1
@show x;

Notice that if does not introduce new scope

if true 
  branch_taken = true 
  branch_not_taken = true 

Let's assume a function, which takes as a first argument a function

  g(f::Function, args...) = println("f called on $(args) evaluates to ", f(args...))

We can use g as g(+, 1, 2), or with a lambda function g(x -> x^2, 2). But sometimes, it might be useful to the lambda function to span multiple lines. This can be achieved by a do block as

g(1,2,3) do a,b,c
  a*b + c


Package is a source tree with a standard layout. It provides a module and thus can be loaded with include or using.

Minimimal package:

├── src/
│   └── PackageName.jl
├── Project.toml


  • Project.toml file describing basic properties:

    • Name, does not have to be unique (federated package sources)
    • UUID, has to be unique (generated automatically)
    • optionally [deps], [targets],...
  • file src/PackageName.jl that defines module PackageName which is executed when loaded.

Many other optional directories:

  • directory tests/, (almost mandatory)
  • directory docs/ (common)
  • directory scripts/, examples/,... (optional)

"Type piracy" refers to the practice of extending or redefining methods in Base or other packages on types that you have not defined. In extreme cases, you can crash Julia (e.g. if your method extension or redefinition causes invalid input to be passed to a ccall). Type piracy can complicate reasoning about code, and may introduce incompatibilities that are hard to predict and diagnose.

module A
import Base.*
*(x::Symbol, y::Symbol) = Symbol(x,y)

The package typically loads other modules that form package dependencies.

Project environments

Is a package that does not contain Name and UUID in Project.toml. It's used when you don't need to create a package for your work. It's created by activate some/path in REPL package mode.

Project Manifest

Both package and environment can contain an additional file Manifest.toml. This file tracks full dependency tree of a project including versions of the packages on which it depends.

for example:

# This file is machine-generated - editing it directly is not advised

deps = ["LinearAlgebra"]
git-tree-sha1 = "485ee0867925449198280d4af84bdb46a2a404d0"
uuid = "621f4979-c628-5d54-868e-fcf4e3e8185c"
version = "1.0.1"

git-tree-sha1 = "03e0550477d86222521d254b741d470ba17ea0b5"
uuid = "1520ce14-60c1-5f80-bbc7-55ef81b5835c"
version = "0.3.4"

Content of files Project.toml and Manifest.toml are maintained by PackageManager.

Package manager

Handles both packages and projects:

  • creating a project ]generate PkgName

  • adding an existing project add PkgName or add

    Names are resolved by Registrators (public or private).

  • removing ]rm PkgName

  • updating ]update

  • developing ]dev http://...

    • add treats packages as being finished, version handling pkg manager. Precompiles!
    • dev leaves all operations on the package to the user (git versioning, etc.). Always read content of files

By default these operations are related to environment .julia/environments/v1.9

E.g. running and updating will update packages in Manifest.toml in this directory. What if the update breaks functionality of some project package that uses special features?

There can and should be more than one environment!

Project environments are based on files with installed packages.

  • switching by ]activate Path - creates Project.toml if not existing

  • from that moment, all package modifications will be relevant only to this project!

  • when switching to a new project ]instantiate will prepare (download and precompile) the environment

    • creates Manifest.toml = list of all exact versions of all packages
  • which Packages are visible is determined by LOAD_PATH

    • typically contains default libraries and default environment
    • it is different for REPL and Pkg.tests ! No default env. in tests.

Package hygiene - workflow

Potential danger

Package dependencies may not be compatible:

  • package A requires C@<0.2
  • package B requires C@>0.3
  • what should happen when ]add A and add B?
  • keep your "@v#.#" as clean as possible (recommended are only debugging/profiling packages)
  • use packages as much as you can, even for short work with scripts ]activate .
    • adding a package existing elsewhere is cheap (global cache)
  • if do you not wish to store any files just test random tricks of a cool package: ]activate --temp

Package development with Revise

Developing a package with interactive test/development:

  1. Create a package/module at one directory MainPackage
  2. Create a script at another directory MainScript, and activate it ]activate .
  3. dev MainPackage in the MainScript environment
    • Revise.jl will watch the MainPackage so it is always up to date
    • in dev mode you have full control over commits etc.

Package Extensions

Some functionality of a package that depends on external packages may not be always needed. A typical example is plotting and visualization that may reguire heavy visualization packages. These are completely unnecessary e.g. in distributed server number crunching.

The extension is a new module depending on: i) the base package, and ii) the conditioning package. It will not be compiled if the conditioning package is not loaded. Once the optional package is loaded, the extension will be automatically compiled and loaded.

New feature since Julia 1.9:

  • new directory in project tree: ext/ the extending module is stored here
  • new section in Project.toml called [extensions] listing extension names and their conditioning packages

Unit testing, /test

Without explicit keywords for checking constructs (think missing functions in interfaces), the good quality of the code is guaranteed by detailed unit testing.

  • each package should have directory /test

  • file /test/runtest.jl is run by the command ]test of the package manager

    this file typically contains include of other tests

  • no formal structure of tests is prescribed

    • test files are just ordinary julia scripts
    • user is free to choose what to test and how (freedom x formal rules)
  • testing functionality is supported by macros @test and @teststet

    @testset "trigonometric identities" begin
        θ = 2/3*π
        @test sin(-θ) ≈ -sin(θ)
        @test cos(-θ) ≈ cos(θ)
        @test sin(2θ) ≈ 2*sin(θ)*cos(θ)
        @test cos(2θ) ≈ cos(θ)^2 - sin(θ)^2

Testset is a collection of tests that will be run and summarized in a common report.

  • Testsets can be nested: testsets in testsets

  • tests can be in loops or functions

    for i=1:10
       @test a[i]>0
  • Useful macro checks for equality with given tolerance

    @test a≈5
    @test a≈5 atol=1e-10
  • @testset resets RNG to Random.GLOBAL_SEED before and after the test for repeatability

    The same results of RNG are not guaranteed between Julia versions!

  • Test coverage: package Coverage.jl

  • Can be run automatically by continuous integration, e.g. GitHub actions

  • integration in VSCode test via package TestItems.jl

Documentation & Style, /docs

A well written package is reusable if it is well documented.

The simpliest kind of documentation is the docstring:

"Auxiliary function for printing a hello"

More complex function that adds π to input:
- x is the input argument (itemize)

Can be written in latex: ``x \leftarrow x + \pi``
addπ(x) = x+π


Renders as

More complex function that adds π to input:

  • x is the input argument (itemize)

Can be written in latex: $x \leftarrow x + \pi$

Structure of the document

├── src/
│   └── SourceFile.jl
├── docs/
│    ├── build/
│    ├── src/
│    └── make.jl

Where the line-by-line documentation is in the source files.

  • /docs/src folder can contain more detailed information: introductory pages, howtos, tutorials, examples
  • running make.jl controls which pages are generated in what form (html or latex) documentation in the /build directory
  • automated with GitHub actions

Documentation is generated by the julia code.

  • code in documentation can be evaluated
julia> x=33
julia> @show xx = 3 3
  • documentation can be added by code:

    struct MyType
    Docs.getdoc(t::MyType) = "Documentation for MyType with value $(t.value)"
    x = MyType("x")
    y = MyType("y")

    See ?x and ?y.

    It uses the same very standard building blocks: multiple dispatch.


By default, every package is precompiled when loading and stored in compiled form in a cache.

If it defines methods that extend previously defined (e.g. from Base), it may affect already loaded packages which need to be recompiled as well. May take time.

Julia has a tracking mechanism that stores information about the whole graph of dependencies.

Faster code can be achieved by the precompile directive:

module FSum

fsum(x) = x
fsum(x,p...) = x+fsum(p...)


Can be investigated using MethodAnalysis.

using MethodAnalysis
mi =methodinstances(fsum)

Useful packages:

  • PackageCompiler.jl has three main purposes:

    • Creating custom sysimages for reduced latency when working locally with packages that has a high startup time.
    • Creating "apps" which are a bundle of files including an executable that can be sent and run on other machines without Julia being installed on that machine.
    • Creating a relocatable C library bundle form of Julia code.
  • AutoSysimages.jl allows easy generation of precompiles images - reduces package loading

Additional material