Julia Language Cross-Version Compatibility Using Compat.jl
Example
The Compat.jl package enables using some new Julia features and syntax with older versions of Julia. Its features are documented on its README, but a summary of useful applications is given below.
0.5.0
Unified String type
In Julia v0.4, there were many different types of strings. This system was considered overly complex and confusing, so in Julia v0.5, there remains only the String type. Compat allows using the String type and constructor on version 0.4, under the name Compat.String. For example, this v0.5 code
buf = IOBuffer()
println(buf, "Hello World!")
String(buf) # "Hello World!\n"
can be directly translated to this code, which works on both v0.5 and v0.4:
using Compat
buf = IOBuffer()
println(buf, "Hello World!")
Compat.String(buf) # "Hello World!\n"
Note that there are some caveats.
- On v0.4,
Compat.Stringis typealiased toByteString, which isUnion{ASCIIString, UTF8String}. Thus, types withStringfields will not be type stable. In these situations,Compat.UTF8Stringis advised, as it will meanStringon v0.5, andUTF8Stringon v0.4, both of which are concrete types. - One has to be careful to use
Compat.Stringorimport Compat: String, becauseStringitself has a meaning on v0.4: it is a deprecated alias forAbstractString. A sign thatStringwas accidentally used instead ofCompat.Stringis if at any point, the following warnings appear:
WARNING: Base.String is deprecated, use AbstractString instead.
likely near no file:0
WARNING: Base.String is deprecated, use AbstractString instead.
likely near no file:0
Compact broadcasting syntax
Julia v0.5 introduces syntactic sugar for broadcast. The syntax
f.(x, y)
is lowered to broadcast(f, x, y). Examples of using this syntax include sin.([1, 2, 3]) to take the sine of multiple numbers at once.
On v0.5, the syntax can be used directly:
julia> sin.([1.0, 2.0, 3.0])
3-element Array{Float64,1}:
0.841471
0.909297
0.14112
However, if we try the same on v0.4, we get an error:
julia> sin.([1.0, 2.0, 3.0])
ERROR: TypeError: getfield: expected Symbol, got Array{Float64,1}
Luckily, Compat makes this new syntax usable from v0.4 also. Once again, we add using Compat. This time, we surround the expression with the @compat macro:
julia> using Compat
julia> @compat sin.([1.0, 2.0, 3.0])
3-element Array{Float64,1}:
0.841471
0.909297
0.14112
