Revise.jl may help you keep your sessions running longer, reducing the need to restart Julia whenever you make changes to code. With Revise, you can be in the middle of a session and then issue a Pkg.update() and/or edit the source code; typically, the changes will be incorporated into the very next command you issue from the REPL. This can save you the overhead of restarting, loading packages, and waiting for code to JIT-compile.

julia> Pkg.add("Example")
INFO: Installing Example v0.4.1
INFO: Package database updated

julia> using Revise        # importantly, this must come before `using Example`

julia> using Example

julia> hello("world")
"Hello, world"

julia> Example.f()
ERROR: UndefVarError: f not defined

julia> edit("Example.jl")  # add a function `f() = π` and save the file

julia> Example.f()
π = 3.1415926535897...

To a limited extent, it's even possible to use Revise on code in Julia's Base module: just say Revise.track(Base). You'll see warnings about some files that are not tracked (see more information below).

By default, Revise processes any modified source files every time you enter a command at the REPL. However, there might be times where you'd prefer to exert manual control over the timing of revisions. Revise looks for an environment variable JULIA_REVISE, and if it is set to anything other than "auto" it will require that you manually call revise() to update code.

For example, on Linux you can start your Julia session as

$ JULIA_REVISE=manual julia

and then revisions will be processed only when you call revise(). If you prefer this mode of operation, you can add that variable to your bash environment or add ENV["JULIA_REVISE"] = "manual" to your .juliarc.jl before you say using Revise (see below).

If you like Revise, you can ensure that every Julia session uses it by adding the following to your .juliarc.jl file:

@schedule begin
    sleep(0.1)
    @eval using Revise
end

This should work the REPL, Juno, and IJulia. For VSCode see these instructions.

Revise is based on the fact that you can change functions even when they are defined in other modules. Here's an example showing how you do that manually (without using Revise):

julia> convert(Float64, π)
3.141592653589793

julia> # That's too hard, let's make life easier for students

julia> eval(Base, quote
       convert(::Type{Float64}, x::Irrational{:π}) = 3.0
       end)
WARNING: Method definition convert(Type{Float64}, Base.Irrational{:π}) in module Base at irrationals.jl:130 overwritten at REPL[2]:2.
convert (generic function with 700 methods)

julia> convert(Float64, π)
3.0

Revise removes some of the tedium of manually copying and pasting code into eval statements. To decrease the amount of re-JITting required, Revise avoids reloading the entire package; instead, it takes care to eval only the changes in your package(s), much as you would if you were doing it manually.

To accomplish this, Revise uses the following overall strategy:

• add a callback to Base so that Revise gets notified when new packages are loaded
• parse the source code for packages when they are first loaded. This allows Revise to determine the module associated with each line of code, and assemble a list of included files for the package. Revise then caches the parsed code so that it is possible to detect changes in the future.
• monitor the file system for changes to any of the included files; it immediately appends any updates to a list of file names that need future processing
• intercept the REPL's backend to ensure that the list of files-to-be-revised gets processed each time you execute a new command at the REPL
• when a revision is triggered, the source file(s) are re-parsed, and a diff between the cached version and the new version is created. eval the diff in the appropriate module(s).
• replace the cached version of each source file with the new version.

Revise only tracks files that have been required as a consequence of a using or import statement; files loaded by include are not tracked, unless you explicitly use Revise.track(filename).

There are some kinds of changes that Revise cannot incorporate into a running Julia session:

• changes to type definitions
• function or method deletions
• file or module renames
• changes to macros that affect method definitions, or to functions that affect generated function expansion. To work around this issue, you may explicitly call revise(module) to force reevaluating every definition in module.
• changes in files that are omitted by Revise (you should see a warning about these). Revise has to be able to statically parse the paths in your package; statements like include("file2.jl") are easy but include(string((length(Core.ARGS)>=2 ? Core.ARGS[2] : ""), "build_h.jl")) cannot be handled.

These kinds of changes require that you restart your Julia session.

Revise became possible because of Jameson Nash's fix of Julia issue 265.