5. Creating Packages

5. Creating Packages

A package is a project with a name, uuid and version entry in the Project.toml file, and a src/PackageName.jl file that defines the module PackageName. This file is executed when the package is loaded.

Generating files for a package

To generate files for a new package, use pkg> generate.

(v1.0) pkg> generate HelloWorld

This creates a new project HelloWorld with the following files (visualized with the external tree command):

shell> cd HelloWorld

shell> tree .
├── Project.toml
└── src
    └── HelloWorld.jl

1 directory, 2 files

The Project.toml file contains the name of the package, its unique UUID, its version, the author and eventual dependencies:

name = "HelloWorld"
uuid = "b4cd1eb8-1e24-11e8-3319-93036a3eb9f3"
version = "0.1.0"
author = ["Some One <someone@email.com>"]


The content of src/HelloWorld.jl is:

module HelloWorld

greet() = print("Hello World!")

end # module

We can now activate the project and load the package:

pkg> activate .

julia> import HelloWorld

julia> HelloWorld.greet()
Hello World!

Adding dependencies to the project

Let’s say we want to use the standard library package Random and the registered package JSON in our project. We simply add these packages (note how the prompt now shows the name of the newly generated project, since we are inside the HelloWorld project directory):

(HelloWorld) pkg> add Random JSON
 Resolving package versions...
  Updating "~/Documents/HelloWorld/Project.toml"
 [682c06a0] + JSON v0.17.1
 [9a3f8284] + Random
  Updating "~/Documents/HelloWorld/Manifest.toml"
 [34da2185] + Compat v0.57.0
 [682c06a0] + JSON v0.17.1
 [4d1e1d77] + Nullables v0.0.4

Both Random and JSON got added to the project’s Project.toml file, and the resulting dependencies got added to the Manifest.toml file. The resolver has installed each package with the highest possible version, while still respecting the compatibility that each package enforce on its dependencies.

We can now use both Random and JSON in our project. Changing src/HelloWorld.jl to

module HelloWorld

import Random
import JSON

greet() = print("Hello World!")
greet_alien() = print("Hello ", Random.randstring(8))

end # module

and reloading the package, the new greet_alien function that uses Random can be used:

julia> HelloWorld.greet_alien()
Hello aT157rHV

Adding a build step to the package.

The build step is executed the first time a package is installed or when explicitly invoked with build. A package is built by executing the file deps/build.jl.

shell> cat deps/build.jl
println("I am being built...")

(HelloWorld) pkg> build
  Building HelloWorld → `deps/build.log`
 Resolving package versions...

shell> cat deps/build.log
I am being built...

If the build step fails, the output of the build step is printed to the console

shell> cat deps/build.jl

(HelloWorld) pkg> build
  Building HelloWorld → `deps/build.log`
 Resolving package versions...
┌ Error: Error building `HelloWorld`:
│ ERROR: LoadError: Ooops
│ Stacktrace:
│  [1] error(::String) at ./error.jl:33
│  [2] top-level scope at none:0
│  [3] include at ./boot.jl:317 [inlined]
│  [4] include_relative(::Module, ::String) at ./loading.jl:1071
│  [5] include(::Module, ::String) at ./sysimg.jl:29
│  [6] include(::String) at ./client.jl:393
│  [7] top-level scope at none:0
│ in expression starting at /Users/kristoffer/.julia/dev/Pkg/HelloWorld/deps/build.jl:1
└ @ Pkg.Operations Operations.jl:938

Adding tests to the package

When a package is tested the file test/runtests.jl is executed.

shell> cat test/runtests.jl
(HelloWorld) pkg> test
   Testing HelloWorld
 Resolving package versions...
   Testing HelloWorld tests passed

Test-specific dependencies

Sometimes one might want to use some packages only at testing time but not enforce a dependency on them when the package is used. This is possible by adding dependencies to [extras] and a test target in [targets] to the Project file. Here we add the Test standard library as a test-only dependency by adding the following to the Project file:

Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"

test = ["Test"]

We can now use Test in the test script and we can see that it gets installed on testing:

shell> cat test/runtests.jl
using Test
@test 1 == 1

(HelloWorld) pkg> test
   Testing HelloWorld
 Resolving package versions...
  Updating `/var/folders/64/76tk_g152sg6c6t0b4nkn1vw0000gn/T/tmpPzUPPw/Project.toml`
  [d8327f2a] + HelloWorld v0.1.0 [`~/.julia/dev/Pkg/HelloWorld`]
  [8dfed614] + Test
  Updating `/var/folders/64/76tk_g152sg6c6t0b4nkn1vw0000gn/T/tmpPzUPPw/Manifest.toml`
  [d8327f2a] + HelloWorld v0.1.0 [`~/.julia/dev/Pkg/HelloWorld`]
   Testing HelloWorld tests passed```

Package naming guidelines

Package names should be sensible to most Julia users, even to those who are not domain experts. The following guidelines applies to the General registry, but may be useful for other package registries as well.

Since the General registry belongs to the entire community people may have opinions about your package name when you publish it, especially if it's ambiguous or can be confused with something other than what it is. Usually you will then get suggestions for a new name that may fit your package better.

  1. Avoid jargon. In particular, avoid acronyms unless there is minimal possibility of confusion.

    • It's ok to say USA if you're talking about the USA.
    • It's not ok to say PMA, even if you're talking about positive mental attitude.
  2. Avoid using Julia in your package name.

    • It is usually clear from context and to your users that the package is a Julia package.
    • Having Julia in the name can imply that the package is connected to, or endorsed by, contributors to the Julia language itself.
  3. Packages that provide most of their functionality in association with a new type should have pluralized names.

    • DataFrames provides the DataFrame type.
    • BloomFilters provides the BloomFilter type.
    • In contrast, JuliaParser provides no new type, but instead new functionality in the JuliaParser.parse() function.
  4. Err on the side of clarity, even if clarity seems long-winded to you.

    • RandomMatrices is a less ambiguous name than RndMat or RMT, even though the latter are shorter.
  5. A less systematic name may suit a package that implements one of several possible approaches to its domain.

    • Julia does not have a single comprehensive plotting package. Instead, Gadfly, PyPlot, Winston and other packages each implement a unique approach based on a particular design philosophy.
    • In contrast, SortingAlgorithms provides a consistent interface to use many well-established sorting algorithms.
  6. Packages that wrap external libraries or programs should be named after those libraries or programs.

    • CPLEX.jl wraps the CPLEX library, which can be identified easily in a web search.
    • MATLAB.jl provides an interface to call the MATLAB engine from within Julia.