The Compat package is designed to ease interoperability between older and newer versions of the Julia language. In particular, in cases where it is impossible to write code that works with both the latest Julia master branch and older Julia versions, or impossible to write code that doesn't generate a deprecation warning in some Julia version, the Compat package provides a macro that lets you use the latest syntax in a backwards-compatible way.

This is primarily intended for use by other Julia packages, where it is important to maintain cross-version compatibility.

To use Compat in your Julia package, add it as a dependency of your package using the package manager

pkg> add Compat

and add a version specifier line such as Compat = "3.22, 4" in the [compat]section of the Project.toml file in your package directory. The version in the latter should be the minimum version that supports all needed features (see list below). Note that Compat v4 requires Julia v1.6, but some features may have been backported to Compat v3 (see the feature list of the release-3 branch). If you require any of those backported features, be sure to specify the correct compatibility in your Project.toml. E.g. if the feature from Compat v4.x has been backported to v3.y, use Compat = 3.y, 4.x. If you use a feature that had originally been added in Compat v3 (e.g. in 3.x), don't forget to also declare compatibility with v4 with Compat = 3.x, 4 (unless you use one the very few things that got removed between Compat v3 and v4, which you most probably don't).

To minimize dependency conflicts between packages it is recommended that packages allow for both appropriate v4 and v3 versions of Compat.jl in their Project.toml (except for rare cases of packages that support only v4 or v3 version of Compat.jl).

Then, in your package, shortly after the module statement include a line like this:

using Compat

and then as needed add

@compat ...compat syntax...

wherever you want to use syntax that differs in the latest Julia master (the development version of Julia). The compat syntax is usually the syntax on Julia master. However, in a few cases where this is not possible, a slightly different syntax might be used. Please check the list below for the specific syntax you need.

Features in the development versions of julia may be added and released in Compat.jl. However, such features are considered experimental until the relevant julia version is released. These features can be changed or removed without incrementing the major version of Compat.jl if necessary to match changes in julia.

• allequal(f, itr) and allunique(f, itr) methods. (#47679) (since Compat 4.13.0)

• logrange(lo, hi; length) is like range but with a constant ratio, not difference. (#39071) (since Compat 4.14.0) Note that on Julia 1.8 and earlier, the version from Compat has slightly lower floating-point accuracy than the one in Base (Julia 1.11 and later).

• Iterators.cycle(itr, n) is the lazy version of repeat(vector, n). (#47354) (since Compat 4.13.0)

• @compat public foo, bar marks foo and bar as public in Julia 1.11+ and is a no-op in Julia 1.10 and earlier. (#50105) (since Compat 3.47.0, 4.10.0)

• redirect_stdio, for simple stream redirection. (#37978) (since Compat 4.8.0)

• trunc, floor, ceil, and round to Bool. (#25085) (since Compat 4.7.0)

• splat(f) which is equivalent to args -> f(args...). (#42717, #48038) (since Compat 4.6.0) (Note: for historical reasons, Compat on Julia before v1.9 also exports Splat; its usage is discouraged, however.)

• Compat.@assume_effects setting... ex overrides the compiler's effect modeling for the method definition ex on Julia versions that support this feature. Julia version without support just pass back ex. (#43852) (since Compat 4.4.0)

• div, lcm, gcd, /, rem, and mod will promote heterogenous Dates.Periods (@bdf9ead9). (since Compat 4.3.0)

• stack combines a collection of slices into one array (#43334). (since Compat 3.46.0, 4.2.0)

• keepat! removes the items at all the indices which are not given and returns the modified source (#36229, #42351). (since Compat 3.44.0, 4.1.0)

• @compat (; a, b) = (; c=1, b=2, a=3) supports property descturing assignment syntax (#39285).

• allequal, the opposite of allunique (#43354). (since Compat 3.42.0)

• eachsplit for iteratively performing split(str). (#39245). (since Compat 3.41.0)

• ismutabletype(t::Type) check whether a type is mutable (the field mutable of DataType was removed. #39037) (since Compat 3.40)

• convert(::Type{<:Period}, ::CompoundPeriod) can convert CompoundPeriods into the specified Period type (#40803) (since Compat 3.38.0)

• Compat.@inline and Compat.@noinline can be used at function callsites to encourage the compiler to (not) inline the function calls on Julia versions that support these features, and otherwise do not have any effects (#41312) (since Compat 3.37)

• Compat.@inline and Compat.@noinline can be used within function body to hint to the compiler the inlineability of the defined function (#41312) (since Compat 3.37)

• Compat.@constprop :aggressive ex and Compat.@constprop :none ex allow control over constant-propagation during inference on Julia versions that support this feature, and otherwise just pass back ex. (#42125) (since Compat 3.36)

• Returns(value) returns value for any arguments (#39794) (since Compat 3.35)

• The function current_exceptions() has been added to get the current exception stack. Julia-1.0 lacks runtime support for full execption stacks, so we return only the most recent exception in that case. (#29901) (since Compat 3.34)

• Two argument methods findmax(f, domain), argmax(f, domain) and the corresponding min versions (#35316, #41076) (since Compat 3.31.1)

• isunordered(x) returns true if x is value that is normally unordered, such as NaN or missing (#35316) (since Compat 3.31.1)

• get accepts tuples and numbers (#41007, #41032) (since Compat 3.31)

• @something and @coalesce as short-circuiting versions of something and coalesce (#40729) (since Compat 3.29)

• pkgversion(m::Module) returns the version of the package that loaded a given module (#45607) (since Compat 4.11)

• VersionNumber(::VersionNumber) defined as a no-op constructor (#45052) (since Compat 4.12)

One of the most important rules for Compat.jl is to avoid breaking user code whenever possible, especially on a released version.

Although the syntax used in the most recent Julia version is the preferred compat syntax, there are cases where this shouldn't be used. Examples include when the new syntax already has a different meaning on previous versions of Julia, or when functions are removed from Base Julia and the alternative cannot be easily implemented on previous versions. In such cases, possible solutions are forcing the new feature to be used with qualified name in Compat.jl (e.g. use Compat.<name>) or reimplementing the old features on a later Julia version.

If you're adding additional compatibility code to this package, the contrib/commit-name.sh script in the base Julia repository is useful for extracting the version number from a git commit SHA. For example, from the git repository of julia, run something like this:

bash $ contrib/commit-name.sh a378b60fe483130d0d30206deb8ba662e93944da
0.5.0-dev+2023

This prints a version number corresponding to the specified commit of the form X.Y.Z-aaa+NNNN, and you can then test whether Julia is at least this version by VERSION >= v"X.Y.Z-aaa+NNNN".

Note that you should specify the correct minimum version for Compat in the [compat] section of your Project.toml, as given in above list.