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 a line Compat to the REQUIRE file in your package directory. Then, in your package, shortly after the module statement include lines like these:

using Compat
import Compat.String

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.

Currently, the @compat macro supports the following syntaxes:

• @compat foo.:bar - foo.(:bar) in 0.4 (#15032).

• @compat f.(args...) - broadcast(f, args...) in 0.4 (#15032).

• @compat (a::B{T}){T}(c) = d - the Julia 0.5-style call overload.

• @compat(get(io, s, false)), with s equal to :limit, :compact or :multiline, to detect the corresponding print settings (performs useful work only on Julia 0.5, defaults to false otherwise)

• @compat import Base.show and @compat function show(args...) for handling the deprecation of writemime in Julia 0.5 ([#16563]). See JuliaLang#219.

• @compat @boundscheck checkbounds(...) rewrites to unconditionally call checkbounds(...) in 0.4. The 0.4-style two-argument form of @boundscheck is left unchanged.

• @compat Nullable(value, hasvalue) to handle the switch from the Nullable :isnull field to :hasvalue field. #18510

• @compat x .= y converts to an in-place assignment to x (via broadcast!) #17510. However, beware that .= in Julia 0.4 has the precedence of ==, not of assignment =, so if the right-hand-side y includes expressions with lower precedence than == you should enclose it in parentheses x .= (y) to ensure the correct order of evaluation.

• String has undergone multiple changes: in Julia 0.3 it was an abstract type and then got renamed to AbstractString in 0.4; in 0.5, ASCIIString and ByteString were deprecated, and UTF8String was renamed to the (now concrete) type String.

  Compat provides unexported Compat.UTF8String and Compat.ASCIIString type aliases which are equivalent to the same types from Base on Julia 0.4, but to String on Julia 0.5. In most cases, using these types by calling import Compat: UTF8String, ASCIIString should be enough. Though note that Compat.ASCIIString does not guarantee that the string only contains ASCII characters on Julia 0.5: call isascii to check if the string is pure ASCII if needed.

  Compat also provides an unexported Compat.String type which is equivalent to ByteString on Julia 0.4, and to String on Julia 0.5. This type should be used only in places where ByteString was used on Julia 0.4, i.e. where either ASCIIString or UTF8String should be accepted. It should not be used as the default type for variables or fields holding strings, as it introduces type-instability in Julia 0.4: use Compat.UTF8String or Compat.ASCIIString instead.

• bytestring has been replaced in most cases with additional String construction methods; for 0.4 compatibility, the usage involves replacing bytestring(args...) with Compat.String(args...). However, for converting a Ptr{UInt8} to a string, use the new unsafe_string(...) method to make a copy or unsafe_wrap(String, ...) to avoid a copy.

• foreach, similar to map but when the return value is not needed (#13744).

• walkdir, returns an iterator that walks the directory tree of a directory. (#13707)

• allunique, checks whether all elements in an iterable appear only once (#15914).

• Base.promote_eltype_op is available as Compat.promote_eltype_op; however, in Julia 0.3, results may be inaccurate.

• normalize and normalize!, normalizes a vector with respect to the p-norm (#13681)

• redirect_stdout, redirect_stderr, and redirect_stdin take an optional function as a first argument, redirect_std*(f, stream), so that one may use do block syntax (as first available for Julia 0.6).

• unsafe_get returns the :value field of a Nullable object without any null-check and has a generic fallback for non-Nullable argument. #18484 (Also, isnull has a generic fallback for non-Nullable argument.)

• transcode converts between UTF-xx string encodings in Julia 0.5 (as a lightweight alternative to the LegacyStrings package), #17323.

• pointer_to_array and pointer_to_string have been replaced with unsafe_wrap(Array, ...) and unsafe_wrap(String, ...) respectively.

• bytestring(::Ptr, ...) has been replaced with unsafe_string.

• super is now supertype #14338

• qr(A, pivot=b) is now qr(A, Val{b}), likewise for qrfact and qrfact!

• readall and readbytes are now readstring and read #14660

• get_bigfloat_precision is now precision(BigFloat), set_precision is setprecision and with_bigfloat_precision is now also setprecision #13232

• get_rounding is now rounding. set_rounding and with_rounding are now setrounding #13232

• Base.tty_size (which was not exported) is now displaysize in Julia 0.5.

• Compat.LinAlg.checksquare #14601

• issym is now issymmetric #15192

• istext is now istextmime #15708

• symbol is now Symbol #16154; use @compat Symbol(...) if you need Julia 0.3 compatibility.

• write(::IO, ::Ptr, len) is now unsafe_write #14766.

• slice is now view#16972 do import Compat.view and then use view normally without the @compat macro.

• fieldoffsets has been deprecated in favor of fieldoffset#14777.

• print_escaped is now another method of escape_string, print_unescaped a method of unescape_string, and print_joined a method of join. #16603

• writemime has been merged into show #16563. Note that to extend this function requires @compat; see the Supported Syntax section for more information.

• $ is now xor or ⊻ #18977.

• num and den are now numerator and denominator #19246.

• takebuf_array is now a method of take!. takebuf_string(io) becomes String(take!(io)) #19088.

• @static has been added #16219.

• @functorize (not present in any Julia version) takes a function (or operator) and turns it into a functor object if one is available in the used Julia version. E.g. something like mapreduce(Base.AbsFun(), Base.MulFun(), x) can now be written as mapreduce(@functorize(abs), @functorize(*), x), and f(::Base.AbsFun()) as f(::typeof(@functorize(abs))), to work across different Julia versions. Func{1} can be written as supertype(typeof(@functorize(abs))) (and so on for Func{2}), which will fall back to Function on Julia 0.5.

• Compat.@blasfunc makes functionality of Base.LinAlg.BLAS.@blasfunc available on older Julia versions

• @__DIR__ has been added #18380

• @vectorize_1arg and @vectorize_2arg are deprecated on Julia 0.6 in favor of the broadcast syntax #17302. Compat.@dep_vectorize_1arg and Compat.@dep_vectorize_2arg are provided so that packages can still provide the deprecated definitions without causing a depwarn in the package itself before all the users are upgraded.

  Packages are expected to use this until all users of the deprecated vectorized function have migrated. These macros will be dropped when the support for 0.6 is dropped from Compat.

• remotecall, remotecall_fetch, remotecall_wait, and remote_do have the function to be executed remotely as the first argument in Julia 0.5. Loading Compat defines the same methods in older versions of Julia. #13338

• Base.FS is now Base.Filesystem #12819. Compat provides an unexported Compat.Filesystem module that is aliased to Base.FS on Julia 0.4 and Base.Filesystem on Julia 0.5.

• mktemp and mktempdir now have variants which take a function as their first argument for automated cleanup. #9017

• cov and cor don't allow keyword arguments anymore. Loading Compat defines compatibility methods for the new API. #13465

• On versions of Julia that do not contain a Base.Threads module, Compat defines a Threads module containing a no-op @threads macro.

• Base.SingleAsyncWork is now Base.AsyncCondition Compat provides an unexported Compat.AsyncCondition type that is aliased to Base.SingleAsyncWork on Julia 0.4 and Base.AsyncCondition on Julia 0.5.

• repeat now accepts any AbstractArray #14082: Compat.repeat supports this new API on Julia 0.4, and calls Base.repeat on 0.5.

• OS_NAME is now Sys.KERNEL. OS information available as is_apple, is_bsd, is_linux, is_unix, and is_windows. 16219

• cholfact, cholfact!, and chol require that input is either Hermitian, Symmetric or that the elements are perfectly symmetric or Hermitian on 0.5. Compat now defines methods for HermOrSym such that using the new methods are backward compatible.

Currently, no new exported types are introduced by Compat.

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".

One of the most frequent problems package developers encounter is finding the right version of Compat to add to their REQUIRE. This is meant to be a guide on how to specify the right lower bound.

• Find the appropriate fix needed for your package from the Compat README. Every function or feature added to Compat is documented in its README, so you are guaranteed to find it.

• Navigate to the blame page of the README by clicking on the README file on GitHub, and then clicking on the blame button which can be found in the top-right corner.

• Now find your fix, and then find the corresponding commit ID of that fix on the left-hand side. Click on the commit ID. This navigates to a page which recorded that particular commit.

• On the top pane, you should find the list of the tagged versions of Compat that includes this fix. Find the minimum version from there.

• Now specify the correct minimum version for Compat in your REQUIRE file by Compat <version>