either.jl

immutable IRef{T}
    value::T
    IRef() = new()
    IRef(value) = new(value)
end

immutable Result{T,S}
    issuccess::Bool
    value::IRef{T}
    error::IRef{S}
    function Result(issuccess, val)
        issuccess ?
            new(issuccess, IRef{T}(val), IRef{S}()) :
            new(issuccess, IRef{T}(), IRef{S}(val))
    end
end

immutable FailedResult end
immutable SuccessResult end

issuccess(e::Result) = e.issuccess
value(e::Result) = e.issuccess ? e.value.value : throw(FailedResult())
error(e::Result) = e.issuccess ? throw(SuccessResult()) : e.error.value


#=
using BenchmarkTools

# Construction
julia> @benchmark Nullable{Float64}()

BenchmarkTools.Trial: 
  memory estimate:  0.00 bytes
  allocs estimate:  0
  --------------
  minimum time:     2.176 ns (0.00% GC)
  median time:      2.257 ns (0.00% GC)
  mean time:        2.245 ns (0.00% GC)
  maximum time:     13.418 ns (0.00% GC)
  --------------
  samples:          10000
  evals/sample:     1000
  time tolerance:   5.00%
  memory tolerance: 1.00%

julia> @benchmark Nullable{Int}(0)
BenchmarkTools.Trial: 
  memory estimate:  0.00 bytes
  allocs estimate:  0
  --------------
  minimum time:     2.532 ns (0.00% GC)
  median time:      2.626 ns (0.00% GC)
  mean time:        2.600 ns (0.00% GC)
  maximum time:     15.280 ns (0.00% GC)
  --------------
  samples:          10000
  evals/sample:     1000
  time tolerance:   5.00%
  memory tolerance: 1.00%

julia> @benchmark Result{Int,Float64}(true, 1)
BenchmarkTools.Trial: 
  memory estimate:  0.00 bytes
  allocs estimate:  0
  --------------
  minimum time:     2.530 ns (0.00% GC)
  median time:      2.624 ns (0.00% GC)
  mean time:        2.599 ns (0.00% GC)
  maximum time:     42.189 ns (0.00% GC)
  --------------
  samples:          10000
  evals/sample:     1000
  time tolerance:   5.00%
  memory tolerance: 1.00%

julia> @benchmark Result{Int,Float64}(false, 1.0)
BenchmarkTools.Trial: 
  memory estimate:  0.00 bytes
  allocs estimate:  0
  --------------
  minimum time:     2.175 ns (0.00% GC)
  median time:      2.259 ns (0.00% GC)
  mean time:        2.256 ns (0.00% GC)
  maximum time:     15.364 ns (0.00% GC)
  --------------
  samples:          10000
  evals/sample:     1000
  time tolerance:   5.00%
  memory tolerance: 1.00%

# accessors

julia> @benchmark get($(Nullable{Int}(0)))
BenchmarkTools.Trial: 
  memory estimate:  0.00 bytes
  allocs estimate:  0
  --------------
  minimum time:     2.177 ns (0.00% GC)
  median time:      2.259 ns (0.00% GC)
  mean time:        2.243 ns (0.00% GC)
  maximum time:     17.017 ns (0.00% GC)
  --------------
  samples:          10000
  evals/sample:     1000
  time tolerance:   5.00%
  memory tolerance: 1.00%

julia> @benchmark value($(Result{Int, Float64}(true,1)))
BenchmarkTools.Trial: 
  memory estimate:  0.00 bytes
  allocs estimate:  0
  --------------
  minimum time:     2.529 ns (0.00% GC)
  median time:      2.642 ns (0.00% GC)
  mean time:        2.610 ns (0.00% GC)
  maximum time:     15.069 ns (0.00% GC)
  --------------
  samples:          10000
  evals/sample:     1000
  time tolerance:   5.00%
  memory tolerance: 1.00%

julia> @benchmark error($(Result{Int, Float64}(false,1.0)))
BenchmarkTools.Trial: 
  memory estimate:  0.00 bytes
  allocs estimate:  0
  --------------
  minimum time:     2.445 ns (0.00% GC)
  median time:      2.632 ns (0.00% GC)
  mean time:        2.613 ns (0.00% GC)
  maximum time:     13.549 ns (0.00% GC)
  --------------
  samples:          10000
  evals/sample:     1000
  time tolerance:   5.00%
  memory tolerance: 1.00%
=#