Julia macro for typed optional function arguments

maybe.jl

# Mark a function argument of any type as optional by generating a Union.
# If a default value is not defined, it assumes "nothing".
#
# Examples:
#   function(x::Int, @maybe y::MyType)
#   function(x::Int, @maybe y::MyType=someval)
macro maybe(argexpr)
    default = :nothing
    if argexpr.head == :(=)
        argexpr, default = argexpr.args
    end
    @assert (argexpr.head == :(::)) "@maybe is for typed function arguments."
    name, ptype = argexpr.args
    deftype = typeof(eval(default))
    Expr(:kw, :($name::Union($deftype, $ptype)), default)
end

test_maybe.jl

using Base.Test

require("maybe.jl")

type TestType
    name::String
end

# Mostly just make sure these actually parse.
function test_maybe1(x::Int, @maybe tt::TestType)
    return 1
end
function test_maybe2(x::Int, @maybe tt::TestType=nothing)
    return 2
end
function test_maybe3(x::Int, @maybe tt::TestType=TestType("foo"))
    return 3
end
function test_maybe4(@maybe x::Vector{Int})
    return 4
end

incrme = 0
test_maybe_expr(@maybe x::Int=incrme) = x

@test length(methods(test_maybe1)) == 2
@test length(methods(test_maybe2)) == 2
@test length(methods(test_maybe3)) == 2
@test length(methods(test_maybe4)) == 2

@test test_maybe1(0, TestType("")) == 1
@test test_maybe1(0) == 1
@test test_maybe2(0, TestType("")) == 2
@test test_maybe2(0) == 2
@test test_maybe3(0, TestType("")) == 3
@test test_maybe3(0) == 3
@test test_maybe4([0]) == 4
@test test_maybe4() == 4

@test test_maybe_expr() == 0
incrme = incrme + 1
@test test_maybe_expr() == 1
incrme = incrme + 1
@test test_maybe_expr() == 2
@test test_maybe_expr(0) == 0