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 = "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.

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.

• @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)

• The methods Base.include(::Function, ::Module, path) and Base.include_string(::Function, ::Module, code[, filename]) have been added. (#34595) (since Compat 3.33)

• Implicit keywords arguments or named tuples is supported using the @compat macro, e.g. @compat f(; x, f.p) and @compat (; x, f.p) is equivalent to f(; x=x, p=f.p) and (; x=x, p=f.p) respectively. (#34331) (since Compat 3.32.0)

• 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)

• muladd(A,B,z) now accepts arrays (#37065) (since Compat 3.30)

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

• NamedTuple(::Pairs) (#37454) (since Compat 3.28)

• UUID(::UUID) and parse(::Type{UUID}, string) (#36018 / #36199) (since Compat 3.27)

• cispi(x) for accurately calculating cis(pi * x) (#38449) (since Compat 3.25)

• startswith(s, r::Regex) and endswith(s, r::Regex) (#29790) (since Compat 3.24)

• sincospi(x) for calculating the tuple (sinpi(x), cospi(x)) (#35816) (since Compat 3.23)

• Dates.canonicalize can now take a Period as an input (#37391) (since Compat 3.22)

• Import renaming is available through the @compat macro, e.g. @compat import LinearAlgebra as LA and @compat import LinearAlgebra: cholesky as c, lu as l. Note: Import renaming of macros is not supported due to differences in parsing behavior (#37396). (since Compat 3.21).

• Compat.parseatom(text::AbstractString, pos::Integer; filename="none") parses a single atom from text starting at index pos. Returns a Tuple consisting of the parsed expression and the index to resume parsing from. (#35243) (since Compat 3.20)

• Compat.parseall(text::AbstractString; filename="none") parses the whole string text and returns the parsed expression. (#35243) (since Compat 3.20)

• mul!(C, A, B, alpha, beta) now performs in-place multiply add (#29634). (since Compat 3.19).

• reinterpret(reshape, T, a::AbstractArray{S}) reinterprets a to have eltype T while inserting or consuming the first dimension depending on the ratio of sizeof(T) and sizeof(S). (#37559). (since Compat 3.18)

• The composition operator ∘ now returns a Compat.ComposedFunction instead of an anonymous function (#37517). (since Compat 3.17)

• New function addenv for adding environment mappings into a Cmd object, returning the new Cmd object (#37244). (since Compat 3.16)

• contains(haystack, needle) and its one argument partially applied form contains(haystack) have been added, it acts like occursin(needle, haystack) (#35132). (since Compat 3.15)

• startswith(x) and endswith(x), returning partially-applied versions of the functions, similar to existing methods like isequal(x) (#35052). (since Compat 3.15)

• Compat.Iterators.map is added. It provides another syntax Iterators.map(f, iterators...) for writing (f(args...) for args in zip(iterators...)), i.e. a lazy map (#34352). (since Compat 3.14)

• takewhle and dropwhile are added in Compat.Iterators (#33437). (since Compat 3.14)

• Curried comparisons !=(x), >=(x), <=(x), >(x), and <(x) are defined as, e.g., !=(x) = y -> y != x (#30915). (since Compat 3.14)

• strides is defined for Adjoint and Transpose (#35929). (since Compat 3.14)

• Compat.get_num_threads() adds the functionality of LinearAlgebra.BLAS.get_num_threads(), and has matching Compat.set_num_threads(n) (#36360). (since Compat 3.13.0)

• @inferred [AllowedType] f(x) is defined (#27516). (since Compat 3.12.0)

• Search a character in a string with findfirst, findnext, findlast and findprev (#31664). (since Compat 3.12.0)

• ∘(f) = f is defined (#34251). (since Compat 3.11.0)

• union supports Base.OneTo (#35577). (since Compat 3.11.0)

• get supports CartesianIndex when indexing AbstractArrays (#30268). (since Compat 3.10.0)

• similar(::PermutedDimsArray) now uses the parent (#35304). (since Compat 3.9.0)

• isdisjoint(l, r) indicates whether two collections are disjoint (#34427). (since Compat 3.9.0)

• mergewith(combine, dicts...) and mergewith!(combine, dicts...) are like merge(combine, dicts...) and merge!(combine, dicts...) but without the restriction that the argument combine must be a Function (#34296). (since Compat 3.9.0).

• @NamedTuple macro for convenient struct-like syntax for declaring NamedTuple types via key::Type declarations (#34548). (since Compat 3.8.0)

• evalpoly(x, (p1, p2, ...)), the function equivalent to @evalpoly(x, p1, p2, ...) (#32753). (since Compat 3.7.0)

• zero(::Irrational) and one now defined (#34773). (since Compat 3.6.0)

• I1:I2, when I1 and I2 are CartesianIndex values, constructs a CartesianIndices iterator (#29440). (Since Compat 3.5.0)

• oneunit(::CartesianIndex) replaces one(::CartesianIndex) (#29442). (Since Compat 3.5.0)

• ismutable return true iff value v is mutable (#34652). (since Compat 3.4.0)

• uuid5 generates a version 5 universally unique identifier (UUID), as specified by RFC 4122 (#28761). (since Compat 3.3.0)

• dot now has a 3-argument method dot(x, A, y) without storing the intermediate result A*y (#32739). (since Compat 3.2.0)

• pkgdir(m) returns the root directory of the package that imported module m (#33128). (since Compat 3.2.0)

• filter can now act on a Tuple #32968. (since Compat 3.1.0)

• Base.Order.ReverseOrdering has a zero arg constructor #33736. (since Compat 3.0.0)

• Function composition now supports multiple functions: ∘(f, g, h) = f ∘ g ∘ h and splatting ∘(fs...) for composing an iterable collection of functions (#33568). (since Compat 3.0.0)

• only(x) returns the one-and-only element of a collection x (#33129). (since Compat 2.2.0)

• mod now accepts a unit range as the second argument (#32628). (since Compat 2.2.0)

• eachrow, eachcol, and eachslice to iterate over first, second, or given dimension of an array (#29749). (since Compat 2.2.0)

• isnothing for testing if a variable is equal to nothing (#29674). (since Compat 2.1.0)

• hasproperty and hasfield (#28850). (since Compat 2.0.0)

• merge methods with one and n NamedTuples (#29259). (since Compat 2.0.0)

• range supporting stop as positional argument (#28708). (since Compat 1.3.0)

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.