More robust append using singletonof

src/BangBang.jl

@@ -26,6 +26,10 @@ using LinearAlgebra
 using Requires
 using ZygoteRules: @adjoint
 
+# Used in NoBang:
+function ismutable end
+function push!! end
+
 include("NoBang/NoBang.jl")
 using .NoBang: ImmutableContainer, singletonof
 

src/NoBang/NoBang.jl

@@ -5,6 +5,8 @@ using Base: ImmutableDict
 using Requires
 using ConstructionBase: constructorof, setproperties
 
+using ..BangBang: push!!, ismutable
+
 if VERSION < v"1.1-"
     using Future: copy!
 end

src/NoBang/base.jl

@@ -31,7 +31,22 @@ _append(xs, ys) = append!(copy(xs), ys)
 _append(xs, ys::Tuple) = push(xs, ys...)
 _append(xs, ys::Pairs{Symbol, <:Any, <:Any, <:NamedTuple}) = push(xs, ys...)
 
-append(xs::AbstractVector, ys::AbstractVector) = vcat(xs, ys)
+append(xs::AbstractVector, ys::AbstractVector) =
+    if constructorof(typeof(xs)) === constructorof(typeof(ys))
+        vcat(xs, ys)
+    elseif !ismutable(xs) && ismutable(ys)
+        vcat(xs, ys)
+    elseif length(ys) == 0
+        xs
+    #=
+    elseif length(ys) == 1
+        push(xs, ys[1])
+    =#
+    else
+        # Not so robust:
+        # append!!(push(xs, ys[1]), @view ys[2:end])
+        foldl(push!!, (@view ys[2:end]), init=push(xs, ys[1]))
+    end
 
 append(xs::ImmutableContainer, ys) = push(xs, ys...)
 

test/test_typedtables.jl

@@ -14,4 +14,17 @@ using TypedTables
     @test tints == Table(a = [1, 3], b = [2, 4])
 end
 
+@testset "append!!" begin
+    tints = Table(a = [1], b = [2])
+    @test append!!(tints, [(a = 3.0, b = 4.0)])::Table == Table(
+        a = [1.0, 3.0],
+        b = [2.0, 4.0],
+    )
+    @test tints == Table(a = [1], b = [2])
+
+    tints = Table(a = [1], b = [2])
+    @test_broken append!!(tints, [(a = 3, b = 4)]) === tints
+    @test_broken tints == Table(a = [1, 3], b = [2, 4])
+end
+
 end  # module