NullableUtils

This package is not currently registered. It can still be installed by calling

julia> Pkg.clone("https://github.com/davidagold/NullableUtils.jl.git")

NullableUtils provides utilities for working with Nullable objects in the Julia language. In particular, it provides a macro -- @lift -- that can be used to call a function f on an argument signature of (Nullable{U1}, Nullable{U2}, ..., Nullable{Un}) if there exists a method of f defined for an argument signature of (U1, U2, ..., Un). This can be useful if one needs to apply such an f to a collection of Nullable arguments but f has no method for argument signatures of (Nullable{U1}, Nullable{U2}, ..., Nullable{Un}). Calling @lift f(x, y) T, where x, y are Nullable objects will return an empty Nullable{T} if either x or y are null and otherwise return Nullable(f(get(x), get(y))). Consider the following example:

julia> using NullableUtils

julia> f(x::Int, y::Int) = x * y
f (generic function with 1 method)

julia> x = Nullable(5); y = Nullable(5); z = Nullable{Int}();

julia> f(x, y)
ERROR: MethodError: no method matching f(::Nullable{Int64}, ::Nullable{Int64})
 in eval(::Module, ::Any) at ./boot.jl:267

julia> @lift f(x, y) Int
Nullable{Int64}(25)

julia> @lift f(x, z) Int
Nullable{Int64}()

The actual code that @lift splices into the AST can be examined by using macroexpand:

julia> macroexpand(:( @lift f(x, y) Int ))
quote  # /Users/David/.julia/v0.5/NullableUtils/src/liftmacro.jl, line 18:
    if isnull(y) || isnull(x) # /Users/David/.julia/v0.5/NullableUtils/src/liftmacro.jl, line 19:
        Nullable{Int}()
    else  # /Users/David/.julia/v0.5/NullableUtils/src/liftmacro.jl, line 21:
        Nullable(f(get(x),get(y)))
    end
end

Thought, he behavior of @lift is simplest when the function call to be lifted is simplest, arbitrarily complex function calls can be passed to the macro:

julia> macroexpand(:( @lift f(x, g(z, y)) + h(z) Int ))
quote  # /Users/David/.julia/v0.5/NullableUtils/src/liftmacro.jl, line 18:
    if (isnull(y) || isnull(z)) || isnull(x) # /Users/David/.julia/v0.5/NullableUtils/src/liftmacro.jl, line 19:
        Nullable{Int}()
    else  # /Users/David/.julia/v0.5/NullableUtils/src/liftmacro.jl, line 21:
        Nullable(f(get(x),g(get(z),get(y))) + h(get(z)))
    end
end

Work on support for if blocks is in the works.