Compat Package for Julia

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.

Usage

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 = "2.2, 3" 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 fatures (see list below), and (if applicable) any newer major versions verified to be compatible. Then, in your package, shortly after the module statement 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.

Compatibility

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.

Supported features

• @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)

Developer tips

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".

Tagging the correct minimum version of Compat

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