make nothing convertible to an empty Nullable.

add help for nothing and Nullable

ref #9364
alternative to c966812

base/nullable.jl

@@ -10,6 +10,7 @@ immutable NullException <: Exception
 end
 
 Nullable{T}(value::T) = Nullable{T}(value)
+Nullable() = Nullable{Union()}()
 
 eltype{T}(::Type{Nullable{T}}) = T
 
@@ -19,6 +20,9 @@ end
 
 convert{T}(::Type{Nullable{T}}, x::T) = Nullable{T}(x)
 
+convert{T}(::Type{Nullable{T}}, ::Void) = Nullable{T}()
+convert(   ::Type{Nullable   }, ::Void) = Nullable{Union()}()
+
 function show{T}(io::IO, x::Nullable{T})
     if x.isnull
         @printf(io, "Nullable{%s}()", repr(T))

doc/stdlib/base.rst

@@ -557,6 +557,12 @@ Syntax
 Nullables
 ---------
 
+.. function:: Nullable(x)
+
+   Wrap value ``x`` in an object of type ``Nullable``, which indicates whether a value is present.
+   ``Nullable(x)`` yields a non-empty wrapper, and ``Nullable{T}()`` yields an empty instance
+   of a wrapper that might contain a value of type ``T``.
+
 .. function:: get(x)
 
    Attempt to access the value of the ``Nullable`` object, ``x``. Returns the

doc/stdlib/constants.rst

@@ -4,6 +4,12 @@
 Constants
 =========
 
+.. data:: nothing
+
+   The singleton instance of type ``Void``, used by convention when there is no value to
+   return (as in a C ``void`` function).
+   Can be converted to an empty ``Nullable`` value.
+
 .. data:: OS_NAME
 
    A symbol representing the name of the operating system. Possible values

test/nullable.jl

@@ -249,3 +249,7 @@ for T in types
     @test isa(convert(Nullable{Number}, Nullable(one(T))), Nullable{Number})
     @test isa(convert(Nullable{Any}, Nullable(one(T))), Nullable{Any})
 end
+
+@test isnull(convert(Nullable, nothing))
+@test isnull(convert(Nullable{Int}, nothing))
+@test isa(convert(Nullable{Int}, nothing), Nullable{Int})