use Some{T} instead of Nullable{T}

base/array.jl

@@ -2434,6 +2434,14 @@ symdiff(a, b, rest...) = symdiff(a, symdiff(b, rest...))
 
 ## replace/replace! ##
 
+
+struct Some{T}
+    value::T
+end
+
+eltype(::Type{Some{T}}) where {T} = T
+get(x::Some) = x.value
+
 # NOTE: to implement replace! and replace for a new type T, it's enough to define:
 # function _replace!(pred::Callable, new::Callable, A::T, count::Int)
 
@@ -2465,9 +2473,8 @@ replace!(A, old_new::Pair...; count::Integer=typemax(Int)) = _replace!(A, eltype
 function _replace!(A, ::Type{K}, count::Integer, old_new #=::Pair...=#) where {K}
     @inline function prednew(x)
         for o_n in old_new
-            first(o_n) == x && return Nullable{K}(last(o_n))
+            first(o_n) == x && return Some{K}(last(o_n))
         end
-        Nullable{K}()
     end
     _replace!(prednew, A, count)
 end
@@ -2491,7 +2498,7 @@ julia> replace!(isodd, A, 0, count=2)
 ```
 """
 replace!(pred::Callable, A, new; count::Integer=typemax(Int)) =
-    _replace!(x->Nullable(new, pred(x)), A, count)
+    _replace!(x -> if pred(x) Some(new) end, A, count)
 
 """
     replace!(prednew::Function, A; [count::Integer])
@@ -2534,8 +2541,8 @@ function _replace!(prednew::Callable, A::AbstractArray, count::Int)
     c = 0
     @inbounds for i in eachindex(A)
         y = prednew(A[i])
-        if !isnull(y)
-            A[i] = unsafe_get(y)
+        if y !== nothing
+            A[i] = get(y)
             c += 1
             c == count && break
         end
@@ -2581,7 +2588,7 @@ julia> replace(isodd, [1, 2, 3, 1], 0, count=2)
 ```
 """
 replace(pred::Callable, A, new; count::Integer=typemax(Int)) =
-    _replace!(x->Nullable(new, pred(x)), copy(A), count)
+    _replace!(x -> if pred(x) Some(new) end, copy(A), count)
 
 """
     replace(prednew::Function, A; [count::Integer])

base/set.jl

@@ -490,8 +490,8 @@ function _replace!(prednew::Callable, A::Union{Associative,AbstractSet}, count::
     c = 0
     for x in A
         y = prednew(x)
-        if !isnull(y)
-            push!(repl, x => unsafe_get(y))
+        if y !== nothing
+            push!(repl, x => get(y))
             c += 1
             c == count && break
         end

test/arrayops.jl

@@ -2238,10 +2238,12 @@ end
     @test_throws BoundsError checkbounds(zeros(2,3,0), 2, 3)
 end
 
+maybe(v, p) = if p Some(v) end
+
 @testset "replace! & replace" begin
     a = [1, 2, 3, 1]
-    @test replace(x->Nullable(2x, iseven(x)), a) == [1, 4, 3, 1]
-    @test replace!(x->Nullable(2x, iseven(x)), a) === a
+    @test replace(x->maybe(2x, iseven(x)), a) == [1, 4, 3, 1]
+    @test replace!(x->maybe(2x, iseven(x)), a) === a
     @test a == [1, 4, 3, 1]
     @test replace(a, 1=>0) == [0, 4, 3, 0]
     @test replace(a, 1=>0, count=1) == [0, 4, 3, 1]
@@ -2250,7 +2252,7 @@ end
 
     d = Dict(1=>2, 3=>4)
     @test replace(x->x.first > 2, d, 0=>0) == Dict(1=>2, 0=>0)
-    @test replace!(x->Nullable(x.first=>2*x.second, x.first > 2), d) === d
+    @test replace!(x->maybe(x.first=>2*x.second, x.first > 2), d) === d
     @test d == Dict(1=>2, 3=>8)
     @test replace(d, (3=>8)=>(0=>0)) == Dict(1=>2, 0=>0)
     @test replace!(d, (3=>8)=>(2=>2)) === d
@@ -2259,8 +2261,8 @@ end
                                                           Dict(2=>2, 0=>0)]
 
     s = Set([1, 2, 3])
-    @test replace(x->Nullable(2x, x>1), s) == Set([1, 4, 6])
-    @test replace(x->Nullable(2x, x>1), s, count=1) in [Set([1, 4, 3]), Set([1, 2, 6])]
+    @test replace(x->maybe(2x, x>1), s) == Set([1, 4, 6])
+    @test replace(x->maybe(2x, x>1), s, count=1) in [Set([1, 4, 3]), Set([1, 2, 6])]
     @test replace(s, 1=>4) == Set([2, 3, 4])
     @test replace!(s, 1=>2) === s
     @test s == Set([2, 3])