Julia (programming language)

Julia is a high-level, high-performance, dynamic programming language. While it is a general-purpose language and can be used to write any application, many of its features are well suited for numerical analysis and computational science.
Distinctive aspects of Julia's design include a type system with parametric polymorphism in a dynamic programming language; with multiple dispatch as its core programming paradigm. Julia supports concurrent, (composable) parallel and distributed computing (with or without using MPI or the built-in corresponding to "OpenMP-style" threads), and direct calling of C and Fortran libraries without glue code. Julia uses a just-in-time (JIT) compiler that is referred to as "just-ahead-of-time" (JAOT) in the Julia community, as Julia compiles all code (by default) to machine code before running it.
Julia is garbage-collected, uses eager evaluation, and includes efficient libraries for floating-point calculations, linear algebra, random number generation, and regular expression matching. Many libraries are available, including some (e.g., for fast Fourier transforms) that were previously bundled with Julia and are now separate.
Several development tools support coding in Julia, such as integrated development environments (e.g. Microsoft's Visual Studio Code, with extensions available adding Julia support to IDEs, e.g. providing debugging and linting support); with integrated tools, e.g. a profiler (and flame graph support available for the built-in one), debugger, and the Rebugger.jl package "supports repeated-execution debugging" and more.
History.
Work on Julia was started in 2009, by Jeff Bezanson, Stefan Karpinski, Viral B. Shah, and Alan Edelman, who set out to create a free language that was both high-level and fast. On 14 February 2012, the team launched a website with a blog post explaining the language's mission. In an interview with "InfoWorld" in April 2012, Karpinski said of the name "Julia": "There's no good reason, really. It just seemed like a pretty name." Bezanson said he chose the name on the recommendation of a friend, then later wrote that Julia stands for "Jeff’s uncommon lisp is automated".
Since the 2012 launch, the Julia community has grown, and "Julia has been downloaded by users at more than 10,000 companies", with over 25,000,000 downloads , up by 87% in a year (other Julia related statistics up by as much as 113%), and is used at more than 1,500 universities, The JuliaCon academic conference for Julia users and developers has been held annually since 2014 with last years 2020 JuliaCon welcoming over 28,900 unique viewers.
Version 0.3 was released in August 2014, version 0.4 in October 2015, version 0.5 in October 2016, and version 0.6 in June 2017. Both Julia 0.7 (a useful release for testing packages, and for knowing how to upgrade them for 1.0) and version 1.0 were released on 8 August 2018. Work on Julia 0.7 was a "huge undertaking" (e.g., because of "entirely new optimizer"), and some changes were made to semantics, e.g. the iteration interface was simplified; and the syntax changed a little (with the syntax now stable, and same for 1.x and 0.7).
The release candidate for Julia 1.0 (Julia 1.0.0-rc1) was released on 7 August 2018, and the final version a day later (and by now, Julia 1.0.x are the oldest versions still supported, having long-term support (LTS); for at least a year). Julia 1.1 was released in January 2019 with, e.g., a new "exception stack" language feature. Bugfix releases are expected roughly monthly, for 1.4.x and 1.0.x and Julia 1.0.1 up to 1.0.5 have followed that schedule. Julia 1.2 was released in August 2019, and it has e.g. some built-in support for web browsers (for testing if running in JavaScript VM), and Julia 1.5 in August 2020 (and with it Julia 1.4.x, 1.3.x, 1.2.x and Julia 1.1.x releases are no longer maintained). Julia 1.3 added e.g. composable multi-threaded parallelism and a binary artifacts system for Julia packages.
Julia 1.4 allowed better syntax for array indexing to handle e.g. 0-based arrays, with for the second element of array A. The memory model was also changed. Minor release 1.4.2 fixed e.g. a Zlib issue, doubling decompression speed.
Julia 1.5 adds record and replay debugging support, for Mozilla's rr tool. It's a big release, with changed behavior in the REPL (soft scope), same as used in Jupyter, but fully compatible for non-REPL code. Most of the thread API was marked as stable, and with this release "arbitrary immutable objects—regardless of whether they have fields that reference mutable objects or not—can now be stack allocated", reducing heap allocations, e.g. codice_1 are no longer allocating. All versions have worked on performance, but especially work on Julia 1.5 targeted so-called "time-to-first-plot" performance, in general, the speed of compilation itself (as opposed to performance of the generated code), and adds tools for developers to improve package loading. Julia 1.6 also improves such performance even more.
Packages that work in Julia 1.0.x should work in 1.1.x or newer, enabled by the forward compatible syntax guarantee. A notable exception was foreign language interface libraries like JavaCall.jl (for JVM languages like Java or Scala) and Rcall.jl (R language) due to some threading-related changes (at a time when all of the threading-functionality in Julia was marked experimental). The issue was especially complicated for Java's JVM, as it has some special expectations around how the stack address space is used. A workaround called was posted for Julia 1.3.0, while a full fix for Java is pending and has no set due date. In addition, JVM versions since Java 11 do not exhibit this problem. Julia 1.6 had a due date that was pushed to end of 2020, and Julia 1.7 is in development.
Julia 1.6 was the largest release since 1.0, faster on many fronts, e.g. introduced parallel precompilation and faster loading of packages, in some cases "50x speedup in load times for large trees of binary artifacts", and is likely to become the next long-term support (LTS) release of Julia. The milestone for 2.0 currently has no set due date.
Julia 1.6.1 has been released, and Julia 1.7 and 1.8 are the next milestones, with the former postponed to June 1, 2021 and as of Julia 1.7 Julia development is back to time-based releases.
Notable uses.
Julia has attracted some high-profile users, from investment manager BlackRock, which uses it for time-series analytics, to the British insurer Aviva, which uses it for risk calculations. In 2015, the Federal Reserve Bank of New York used Julia to make models of the United States economy, noting that the language made model estimation "about 10 times faster" than its previous MATLAB implementation. Julia's co-founders established Julia Computing in 2015 to provide paid support, training, and consulting services to clients, though Julia remains free to use. At the 2017 JuliaCon conference, Jeffrey Regier, Keno Fischer and others announced that the Celeste project used Julia to achieve "peak performance of 1.54 petaFLOPS using 1.3 million threads" on 9300 Knights Landing (KNL) nodes of the Cori II (Cray XC40) supercomputer (then 6th fastest computer in the world). Julia thus joins C, C++, and Fortran as high-level languages in which petaFLOPS computations have been achieved.
Three of the Julia co-creators are the recipients of the 2019 James H. Wilkinson Prize for Numerical Software (awarded every four years) "for the creation of Julia, an innovative environment for the creation of high-performance tools that enable the analysis and solution of computational science problems." Also, Alan Edelman, professor of applied mathematics at MIT, has been selected to receive the 2019 IEEE Computer Society Sidney Fernbach Award "for outstanding breakthroughs in high-performance computing, linear algebra, and computational science and for contributions to the Julia programming language."
Julia Computing and NVIDIA announce "the availability of the Julia programming language as a pre-packaged container on the NVIDIA GPU Cloud (NGC) container registry" with NVIDIA stating "Easily Deploy Julia on x86 and Arm [..] Julia offers a package for a comprehensive HPC ecosystem covering machine learning, data science, various scientific domains and visualization."
Additionally, "Julia was selected by the Climate Modeling Alliance as the sole implementation language for their next generation global climate model. This multi-million dollar project aims to build an earth-scale climate model providing insight into the effects and challenges of climate change."
Julia is used by NASA and the Brazilian INPE for space mission planning and satellite simulation. Another effort is working on an embedded project to control a satellite in space using Julia for attitude control.
Sponsors.
The Julia language became a NumFOCUS Fiscally sponsored project in 2014 in an effort to ensure the project's long-term sustainability. Dr. Jeremy Kepner at MIT Lincoln Laboratory was the founding sponsor of the Julia project in its early days. In addition, funds from the Gordon and Betty Moore Foundation, the Alfred P. Sloan Foundation, Intel, and agencies such as NSF, DARPA, NIH, NASA, and FAA have been essential to the development of Julia. Mozilla, the maker of Firefox web browser, with its research grants for H1 2019, sponsored "a member of the official Julia team" for the project "Bringing Julia to the Browser", meaning to Firefox and other web browsers.
Julia Computing.
Julia Computing, Inc. was founded in 2015 by Viral B. Shah, Deepak Vinchhi, Alan Edelman, Jeff Bezanson, Stefan Karpinski and Keno Fischer.
In June 2017, Julia Computing raised $4.6million in seed funding from General Catalyst and Founder Collective, and in the same month was "granted $910,000 by the Alfred P. Sloan Foundation to support open-source Julia development, including $160,000 to promote diversity in the Julia community" and in December 2019 the company got $1.1million funding from the US government to "develop a neural component machine learning tool to reduce the total energy consumption of heating, ventilation, and air conditioning (HVAC) systems in buildings".
Language features.
Julia is a general-purpose programming language, while also originally designed for numerical/technical computing.
It is also useful for low-level systems programming, as a specification language, and for web programming at both server and client side.
According to the official website, the main features of the language are:
Multiple dispatch (also termed multimethods in Lisp) is a generalization of single dispatch the polymorphic mechanism used in common object-oriented programming (OOP) languages that uses inheritance. In Julia, all concrete types are subtypes of abstract types, directly or indirectly subtypes of the "Any" type, which is the top of the type hierarchy. Concrete types can not themselves be subtyped the way they can in other languages; composition is used instead (see also inheritance vs subtyping).
Julia draws significant inspiration from various dialects of Lisp, including Scheme and Common Lisp, and it shares many features with Dylan, also a multiple-dispatch-oriented dynamic language (which features an ALGOL-like free-form infix syntax rather than a Lisp-like prefix syntax, while in Julia "everything" is an expression), and with Fortress, another numerical programming language (which features multiple dispatch and a sophisticated parametric type system). While Common Lisp Object System (CLOS) adds multiple dispatch to Common Lisp, not all functions are generic functions.
In Julia, Dylan, and Fortress, extensibility is the default, and the system's built-in functions are all generic and extensible. In Dylan, multiple dispatch is as fundamental as it is in Julia: all user-defined functions and even basic built-in operations like codice_2 are generic. Dylan's type system, however, does not fully support parametric types, which are more typical of the ML lineage of languages. By default, CLOS does not allow for dispatch on Common Lisp's parametric types; such extended dispatch semantics can only be added as an extension through the CLOS Metaobject Protocol. By convergent design, Fortress also features multiple dispatch on parametric types; unlike Julia, however, Fortress is statically rather than dynamically typed, with separate compiling and executing phases. The language features are summarized in the following table:
By default, the Julia runtime must be pre-installed as user-provided source code is run. Alternatively, a standalone executable that needs no Julia source code can be built with "PackageCompiler.jl".
Julia's syntactic macros (used for metaprogramming), like Lisp macros, are more powerful than text-substitution macros used in the preprocessor of some other languages such as C, because they work at the level of abstract syntax trees (ASTs). Julia's macro system is hygienic, but also supports deliberate capture when desired (like for anaphoric macros) using the codice_3 construct.
Interaction.
The Julia official distribution includes an interactive command-line read–eval–print loop (REPL), with a searchable history, tab-completion, and dedicated help and shell modes, which can be used to experiment and test code quickly. The following fragment represents a sample session example where strings are concatenated automatically by println:
julia> p(x) = 2x^2 + 1; f(x, y) = 1 + 2p(x)y
julia> println("Hello world!", " I'm on cloud ", f(0, 4), " as Julia supports recognizable syntax!")
Hello world! I'm on cloud 9 as Julia supports recognizable syntax!
The REPL gives user access to the system shell and to help mode, by pressing codice_4 or codice_5 after the prompt (preceding each command), respectively. It also keeps the history of commands, including between sessions. Code that can be tested inside the Julia's interactive section or saved into a file with a codice_6 extension and run from the command line by typing:
 $ julia <filename>
Julia is supported by Jupyter, an online interactive "notebooks" environment, and Pluto.jl, a "reactive notebook" (where notebooks are saved as pure Julia files), a possible replacement for the former kind.
Use with other languages.
Julia is in practice interoperable with many languages (e.g. majority of top 10–20 languages in popular use). Julia's ccall keyword is used to call C-exported or Fortran shared library functions individually, and packages to allow calling other languages e.g. Python, R, MATLAB, Java or Scala. And packages for other languages, e.g. Python (or R or Ruby), i.e. pyjulia, to call "to" Julia.
Julia has support for the latest Unicode 13.0, with UTF-8 used for strings (by default) and for Julia source code (only allowing legal UTF-8 in the latest version), meaning also allowing as an option common math symbols for many operators, such as ∈ for the codice_7 operator.
Julia has packages supporting markup languages such as HTML (and also for HTTP), XML, JSON and BSON, and for databases and web use in general.
Package system.
Julia has a built-in package manager and includes a default registry system. Packages are most often distributed as source code hosted on GitHub, though alternatives can also be used just as well. Packages can also be installed as binaries, using artifacts. Julia's package manager is used to query and compile packages, as well as managing environments. Federated package registries are supported, allowing registries other than the official to be added locally.
Uses.
Julia has been used to perform petascale computing with the Celeste library for sky surveys. Julia is used by BlackRock Engineering analytical platforms.
Implementation.
Julia's core is implemented in Julia and C, together with C++ for the LLVM dependency. The parsing and code-lowering are implemented in FemtoLisp, a Scheme dialect. The LLVM compiler infrastructure project is used as the back end for generation of 64-bit or 32-bit optimized machine code depending on the platform Julia runs on. With some exceptions (e.g., PCRE), the standard library is implemented in Julia. The most notable aspect of Julia's implementation is its speed, which is often within a factor of two relative to fully optimized C code (and thus often an order of magnitude faster than Python or R). Development of Julia began in 2009 and an open-source version was publicized in February 2012.
Current and future platforms.
While Julia has tier 1 macOS support, meaning for Intel-based Macs, support for the brand-new Apple M1-based Macs isn't explicitly specified (and neither for Windows on ARM). Julia is however claimed to work; "ok" (at reduced performance) with Rosetta 2 (that needs to emulate Julia). Work on native full-speed M1 support (i.e. without emulation) is mostly done, and many programs may work if such a build of Julia is used, since all but one Julia tests pass (except for "Too many open files").
Since Julia uses JIT, Julia generates native machine code directly, before a function is first run (i.e. a different approach than compiling to bytecode, that you distribute by default, to be run on a virtual machine (VM), as with e.g. Java/JVM; then translated from the bytecode while running, as done by Dalvik on older versions of Android).
Julia has four support tiers. All IA-32 processors completely implementing the i686 subarchitecture are supported and 64-bit x86-64 (aka amd64), less than about a decade old, are supported. ARMv8 (AArch64) processors are fully supported in first tier, and ARMv7 and ARMv6 (AArch32) are supported with some caveats (lower tier) for Julia 1.0.x and also had official executables for later versions, while 32-bit ARM support was later downgraded to tier 3 (however, unofficial binaries are available for Julia 1.5.1). CUDA (i.e. Nvidia GPUs; implementing PTX) has tier 1 support, with the help of an external package. There are also additionally packages supporting other accelerators, such as Google's TPUs, and AMD's GPUs also have support with e.g. OpenCL; and experimental support for the AMD ROCm stack. Julia's downloads page provides executables (and source) for all the officially supported platforms.
On some platforms, Julia may need to be compiled from source code (e.g., the original Raspberry Pi), with specific build options, which has been done and unofficial pre-built binaries (and build instructions) are available. Julia has been built 
on several ARM platforms. PowerPC (64-bit) has tier 3 support, meaning it "may or may not build".
Julia is now supported in Raspbian while support is better for newer Pis, e.g., those with ARMv7 or newer; the Julia support is promoted by the Raspberry Pi Foundation.
There is also support for web browsers/JavaScript through JSExpr.jl; and the alternative language of web browsers, WebAssembly, has minimal support for several upcoming external Julia projects.
Julia can compile to ARM; thus, in theory, Android apps can be made with the NDK, but for now Julia has been made to run under Android only indirectly, "i.e." with a Ubuntu chroot on Android.