[julia-users] Is it possible to use Nullable with abstract types?

[Steve Cordwell]

Say I want to be able to set the value of an array to zero where a boolean 
mask array is false, then I can define this function:

    zero_mask!(a::AbstractArray, mask::AbstractArray{Bool}) = setindex!(a, 
zero(T), find(!mask))

I have a function that constructs arrays and part of its job is to call 
zero_mask! if a mask is passed to it. I thought I could use Nullable types 
so that there is only one implementation rather than two and have helper 
methods call the do_stuff with the correct arguments.

    function do_stuff(rows, columns, mask::Nullable{AbstractArray{Bool}})
        a = rand(rows, columns)
        isnull(mask) ? nothing : zero_mask!(a)
        a
    end

It would be nice to use do_stuff with just a boolean array, rather than 
specifying it to be Nullable.

    do_stuff(rows, columns, mask::AbstractArray{Bool}) = do_stuff(rows, 
columns, Nullable(mask))

    do_stuff(rows, columns) = do_stuff(rows, columns, 
Nullable{AbstractArray{Bool}}())

However, I get the following error:

    do_stuff(10, 3, rand(Bool, 10, 3))
    ERROR: MethodError: `do_stuff` has no method matching do_stuff(::Int64, 
::Int64, ::Nullable{Array{Bool,2}})

Presumably because I need to define do_study{N}(rows, columns, 
Nullable{Array{Bool,N}}). Is there a way to use Nullable abstract types?

Cheers