How is Matrix defined

Internals & Design
1

I noticed that Matrix prints out that it is an alias. How would one define an alias for a custom type?

julia> Matrix{Int}
Matrix{Int64} (alias for Array{Int64, 2})
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2

Here’s a simple example:

julia> struct MyType{T, N}
       end

julia> const MyAlias{T} = MyType{T, 2}
MyAlias{T} where T (alias for MyType{T, 2} where T)
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3

You can see here how Matrix is defined in the Julia source code.

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4

Note that the const here is unnecessary. Type aliases like this will always be declared as constants.

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5

PR to remove consts from /base/arrays.jl :wink: ?

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6

Way ahead of you :wink: : NFC: remove unnecessary const before type aliases by simeonschaub · Pull Request #40671 · JuliaLang/julia · GitHub

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7

In which Julia version were these const declarations necessary?

8

That’s just not true (on 1.6.1 at least). There is an observable difference between using const and not, and it’s what you’d expect:

julia> T = Int64
Int64

julia> T = Float64
Float64

vs

julia> const T = Int64
Int64

julia> T = Float64
ERROR: invalid redefinition of constant T
Stacktrace:
 [1] top-level scope
   @ REPL[2]:1
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9

I could probably have been a little more precise. What I meant was type aliases declared with TypeVars on the left hand side, like Foo{T} = Bar{T}. That doesn’t hold true for regular assignment of course.

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