70  Julia interfaces

Julia can be used in many different manners. This page describes a few.

70.1 The REPL

Base Julia comes with a REPL package, which provides a means to interact with Julia at the command line.

               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.7.0 (2021-11-30)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia> 2 + 2
4

The julia> prompt is where commands are typed. The return key will send a command to the interpreter and the results are displayed in the REPL terminal.

The REPL has many features for editing, for interacting with the package manager, or interaction with the shell. However it is command-line based, which no support for mouse interaction. For that, other options are available.

70.2 Pluto

The Pluto package provides a notebook interface for interacting with Julia, which has a few idiosyncrasies, as compared to other interfaces.

Pluto is started from the REPL terminal with these two commands:

using Pluto
Pluto.run()

Primarily, the variables in the notebook are reactive, meaning if a variable’s value is modified, all references to that variables are also modified. This reactive nature makes it very easy to see the results of slight modifications and when coupled with HTML controls, allows easy user interfaces to be developed.

As a result, a variable name may only be used once in the top-level scope. (Names can be reused inside functions, which create their own scope and in “let” blocks, a trick used within these notes.) In the notes, subscripting and unicode variants are used for symbols which are typically repurposed (e.g., x or f).

Pluto cells may only contain one command, the result of which is displayed above the cell. This one command can be a begin or let block to join multiple statements.

Pluto has a built-in package management system that manages the installation of packages on demand.

Pluto notebooks can be easily run locally using Pluto.

Pluto notebooks are just .jl scripts, so can easily be shared.

70.3 IJulia

“Project Jupyter exists to develop open-source software, open-standards, and services for interactive computing across dozens of programming languages.” The IJulia package allows Julia to be one of these programming languages. This package must be installed prior to use.

The Jupyter Project provides two web-based interfaces to Julia: the Jupyter notebook and the newer JupyterLab. The binder project use Juptyer notebooks for their primary interface to Julia. To use a binder notebook, follow this link:

launch binder

To run locally, these interfaces are available once IJulia is installed. Since version 1.7, the following commands should do this:

using IJulia
notebook()

Should that not work, then this should as well:

using Pkg
Pkg.add("PyCall")
Pkg.add("IJulia")

The notebook interface has “cells” where one or more commands can be entered.

In IJulia, a block of commands is sent to the kernel (the Julia interpreter) by typing “shift+return” or clicking on a “run” button. The output is printed below a cell, including graphics.

When a cell is evaluating, the leading [] has an asterisk ([*]) showing the notebook is awaiting the results of the calculation.

Once a cell is evaluated, the leading [] has a number inserted (e.g., [1], as in the figure). This number indicates the order of cell evaluation. Once a notebook is interacted with, the state of the namespace need not reflect the top-to-bottom order of the notebook, but rather reflects the order of cell evaluations.

To be specific, a variable like x may be redefined in a cell above where the variable is initially defined and this redefinition will hold the current value known to the interpreter. As well, a notebook, when reloaded, may have unevaluated cells with output showing. These will not influence the state of the kernel until they are evaluated.

When a cell’s commands are evaluated, the last command executed is displayed. If it is desirable that multiple values be displayed, they can be packed into a tuple. This is done by using commas to separate values. IJulia will also display other means to print output (e.g., @show, display, print, …).

To run all cells in a notebook from top to bottom, the “run all” command under the “Cell” menu is available.

If a calculation takes much longer than anticipated, the “kernel” can be interrupted through a menu item of “Kernel”.

If the kernel appears unresponsive, it can be restarted through a menu item of “Kernel”.

Notebooks can be saved (as *.ipynb files) for sharing or for reuse. Notebooks can be printed at HTML pages, and if the proper underlying software is available, as formatted pages.

JupyterLab, a variant, has more features, commonly associated with an integrated development environment (IDE).

70.4 VSCode

Julia for Visual Studio Code provides support for the julia programming language for VS Code. VS Code is an open-sourced code editor supported by Microsoft. VS Code provides a cross-platform interface to Julia geared towards programming within the language.