How does function named (::Type{T}) work in Julia?

New to Julia
1

I encountored syntax like funcion (::Type{T})(...)... and it seems that (::Type{T}) is a function name. Is it correct? Also, if so, could you explain how it works or tell me the corresponding documentation?

I encountored this syntax here. For example, the code below can be found at the lines 82-87.

function (::Type{M})(
vertices::AbstractVector{Point{3, VT}}, faces::AbstractVector{FT}
) where {M <: HMesh, VT, FT <: Face}
msh = PlainMesh{VT, FT}(vertices = vertices, faces = faces)
convert(M, msh)
end

2

See https://docs.julialang.org/en/latest/manual/methods/#Function-like-objects-1

(and maybe this if you’re unfamiliar with Type{T})

The snippet you posted basically defines constructors - it is defining a method for DataType objects which are a subtype of M. However, it doesn’t have to be a constructor in the sense of creating an object:

abstract type M end
struct A <: M end
struct B <: M end

function (::Type{T})(s::String) where {T <: M}
    println("I'm not constructing an object of type ", T, ". Instead I just print: ", s)
end

which gives

julia> A("test")
I'm not constructing an object of type A. Instead I just print: test

julia> B("hello world")
I'm not constructing an object of type B. Instead I just print: hello world
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