add get method to AbstractDimStack (#889)

src/stack/stack.jl

@@ -189,6 +189,10 @@ Base.NamedTuple(s::AbstractDimStack) = NamedTuple(layers(s))
 Base.collect(st::AbstractDimStack) = parent([st[D] for D in DimIndices(st)])
 Base.Array(st::AbstractDimStack) = collect(st)
 Base.vec(st::AbstractDimStack) = vec(collect(st))
+Base.get(st::AbstractDimStack, k::Symbol, default) =
+    haskey(st, k) ? st[k] : default
+Base.get(f::Base.Callable, st::AbstractDimStack, k::Symbol) =
+    haskey(st, k) ? st[k] : f()
 @propagate_inbounds Base.iterate(st::AbstractDimStack) = iterate(st, 1)
 @propagate_inbounds Base.iterate(st::AbstractDimStack, i) =
     i > length(st) ? nothing : (st[DimIndices(st)[i]], i + 1)

test/stack.jl

@@ -16,7 +16,6 @@ da4 = DimArray(cat(4A, 5A, 6A, 7A; dims=3), (x, y, z); name=:extradim)
 
 s = DimStack((da1, da2, da3))
 mixed = DimStack(da1, da2, da4)
-typeof(s)
 
 @testset "constructors" begin
     @test DimStack((one=A, two=2A, three=3A), dimz) ==
@@ -67,6 +66,10 @@ end
     @test eltype(mixed) === @NamedTuple{one::Float64, two::Float32, extradim::Float64}
     @test haskey(s, :one) == true
     @test haskey(s, :zero) == false
+    @test get(mixed, :one, nothing) == mixed.one
+    @test get(mixed, :five, nothing) == nothing
+    @test get(() -> nothing, mixed, :two) == mixed.two
+    @test get(() -> nothing, mixed, :ten) == nothing
     @test size(mixed) === (2, 3, 4) # size is as for Array
     @test size(mixed, Y) === 3
     @test size(mixed, 3) === 4