# Introduction to Revise

Revise.jl may help you keep your Julia sessions running longer, reducing the need to restart when you make changes to code. With Revise, you can be in the middle of a session and then edit source code, update packages, switch git branches, and/or stash/unstash 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.

Using Revise also improves your experience when using the debuggers. Revise will keep track of changed locations of your methods in file, and ensure that the debugger displays the source code of what you're actually debugging.

Many users automatically load Revise on startup. On versions of Julia older than 1.5, this is slightly more involved than just adding using Revise to .julia/config/startup.jl: see Using Revise by default for details.

## Installation

You can obtain Revise using Julia's Pkg REPL-mode (hitting ] as the first character of the command prompt):

(v1.0) pkg> add Revise

or with using Pkg; Pkg.add("Revise").

## Usage example

We'll make changes to Julia's "Example" package (a trivial package designed to illustrate the file and directory organization of typical packages). We have to "develop" it in order to make changes:

(v1.0) pkg> dev Example
[...output related to installation...]


Now we load Revise (if we haven't already done so) and Example:

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

julia> using Example

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

Now we're going to check that the Example module currently lacks a function named f:

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

But say we really want f, so let's add it. You can either navigate to the source code (at .julia/dev/Example/src/Example.jl) in an editor manually, or you can use Julia to open it for you:

julia> edit(hello)   # opens Example.jl in the editor you have configured

Now, add a function f() = π and save the file. Go back to the REPL (the same REPL, don't restart Julia) and try this:

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

Voila! Even though we'd loaded Example before adding this function, Revise noticed the change and inserted it into our running session.

Warning

Revise's first revision has latency of several seconds–it's compiling all of its internal code, which includes a complete Julia interpreter and all of Revise's parse/diff/patch/cache machinery. After your first revision, future revisions will generally be fast enough that they will seem nearly instantaneous. (There are exceptions, but they occur only in specific circumstances, for example when Revise's own code gets invalidated by your changes.)

Now suppose we realize we've made a horrible mistake: that f method will mess up everything, because it's part of a more complicated dispatch process and incorrectly intercepts certain f calls. No problem, just delete f in your editor, save the file, and you're back to this:

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

all without restarting Julia. While you can evalued new methods without Revise using inline evaluation through your IDE, method deletion is just one example of a change that can only be made easily by Revise.

If you need more examples, see Revise usage: a cookbook.

## Other key features of Revise

Revise updates its internal paths when you change versions of a package. To try this yourself, first re-insert that definition of f in the dev version of Example and save the file. Now try toggling back and forth between the dev and released versions of Example:

(v1.0) pkg> free Example   # switch to the released version of Example

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

(v1.0) pkg> dev Example

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

Revise is not tied to any particular editor. (The EDITOR or JULIA_EDITOR environment variables can be used to specify your preference for which editor gets launched by Julia's edit function.)

If you don't want to have to remember to say using Revise each time you start Julia, see Using Revise by default.

## What Revise can track

Revise is fairly ambitious: if all is working, subject to a few Limitations you should be able to track changes to

• any package that you load with import or using
• any script you load with includet (see Configuring the revise mode for important default restrictions on includet)
• any file defining Base julia itself (with Revise.track(Base))
• any of Julia's standard libraries (with, e.g., using Unicode; Revise.track(Unicode))
• any file defining Core.Compiler (with Revise.track(Core.Compiler))

The last one requires that you clone Julia and build it yourself from source.

## Secrets of Revise "wizards"

Revise can assist with methodologies like test-driven development. While it's often desirable to write the test first, sometimes when fixing a bug it's very difficult to write a good test until you understand the bug better. Often that means basically fixing the bug before your write the test. With Revise, you can

• fix the bug while simultaneously developing a high-quality test
• verify that your test passes with the fixed code
• git stash your fix and check that your new test fails on the old code, thus verifying that your test captures the essence of the former bug (if it doesn't fail, you need a better test!)
• git stash pop, test again, commit, and submit

all without restarting your Julia session.

## Other Revise workflows

Revise can be used to perform work when files update. For example, let's say you want to regenerate a set of web pages whenever your code changes. Suppose you've placed your Julia code in a package called MyWebCode, and the pages depend on "file.js" and all files in the "assets/" directory; then

entr(["file.js", "assets"], [MyWebCode]) do
build_webpages(args...)
end

will execute build_webpages(args...) whenever you save updates to the listed files or MyWebCode.

If you want to regenerate the web page as soon as any change is detected, not only in MyWebCode but also in any package tracked by Revise, you can provide the all keyword argument to entr:

entr(["file.js", "assets"]; all=true) do
build_webpages(args...)
end

## Taking advantage of Revise in other packages

To make it easier for other packages to benefit from Revise without needing to add it as a dependency or understand Revise's internals, Revise interfaces with CodeTracking, which is a small package acting as Revise's "query" interface.

## What else do I need to know?

Except in cases of problems (see below), that's it! Revise is a tool that runs in the background, and when all is well it should be essentially invisible, except that you don't have to restart Julia so often.

Revise can also be used as a "library" by developers who want to add other new capabilities to Julia; the sections How Revise works and Developer reference are particularly relevant for them.

## If Revise doesn't work as expected

If Revise isn't working for you, here are some steps to try:

• See Configuration for information on customization options. In particular, some file systems (like NFS) and current users of WSL2 might require special options.
• Revise can't handle all kinds of code changes; for more information, see the section on Limitations.
• Try running test Revise from the Pkg REPL-mode. If tests pass, check the documentation to make sure you understand how Revise should work. If they fail (especially if it mirrors functionality that you need and isn't working), see Debugging problems with paths for one set of suggestions.

If you still encounter problems, please file an issue. Especially if you think Revise is making mistakes in adding or deleting methods, please see the page on Debugging Revise for information about how to attach logs to your bug report.