Declare the size of an array attribute in a type definition

Question

I currently have a type with an array attribute

immutable foo
    a::Int64
    b::Int64
    x::Array{Float64,1} # One dimension array of Float 64, but no length info
end

I Know that the array will always contain 100 Float64 elements. Is there a way to pass this information in the type annotation? Perhaps something similar to the way one can declare the size of an instantiated array like x = Array(Float64, 100) ?

Answer 1

You can enforce invariants with an inner constructor.

immutable Foo
    a::Int64
    b::Int64
    x::Vector{Float64} # Vector is an alias for one-dimensional array

    function Foo(a,b,x)
        size(x,1) != 100 ?
        error("vector must have exactly 100 values") :
        new(a,b,x)
    end
end

And then from REPL:

julia> Foo(1,2,float([1:99]))
ERROR: vector must have exactly 100 values
 in Foo at none:7

julia> Foo(1,2,float([1:100]))
Foo(1,2,[1.0,2.0,3.0,4.0,5.0,6.0,7.0,8.0,9.0,10.0  …  91.0,92.0,93.0,94.0,95.0,96.0,97.0,98.0,99.0,100.0])