allow unstable Chain{Vector} too

src/layers/basic.jl

@@ -27,6 +27,10 @@ julia> m2 = Chain(enc = Chain(Flux.flatten, Dense(10, 5, tanh)),
 julia> m2(x) == (m2[:dec] ∘ m2[:enc])(x)
 true
 ```
+
+For large models, there is a special type-unstable path which can reduce compilation
+times. This can be used by supplying a vector of layers `Chain([layer1, layer2, ...])`.
+This feature is somewhat experimental, beware!
 """
 struct Chain{T}
   layers::T
@@ -36,6 +40,7 @@ struct Chain{T}
     isempty(kw) && return new{Tuple{}}(())
     new{typeof(values(kw))}(values(kw))
   end
+  Chain(xs::AbstractVector) = new{typeof(xs)}(xs)  # unstable path, to help compile times
 end
 
 @forward Chain.layers Base.getindex, Base.length, Base.first, Base.last,
@@ -53,6 +58,13 @@ end
 
 applychain(layers::NamedTuple, x) = applychain(Tuple(layers), x)
 
+function applychain(layers::AbstractVector, x)
+  for f in layers
+    x = f(x)
+  end
+  x
+end
+
 Base.getindex(c::Chain, i::AbstractArray) = Chain(c.layers[i]...)
 Base.getindex(c::Chain{<:NamedTuple}, i::AbstractArray) = 
   Chain(; NamedTuple{Base.keys(c)[i]}(Tuple(c.layers)[i])...)
@@ -64,6 +76,7 @@ function Base.show(io::IO, c::Chain)
 end
 _show_layers(io, layers::Tuple) = join(io, layers, ", ")
 _show_layers(io, layers::NamedTuple) = join(io, ["$k = $v" for (k, v) in pairs(layers)], ", ")
+_show_layers(io, layers::AbstractVector) = (print(io, "["); join(io, layers, ", "); print(io, "]"))
 
 # This is a temporary and naive implementation
 # it might be replaced in the future for better performance

src/layers/show.jl

@@ -14,11 +14,12 @@ for T in [
 end
 
 function _big_show(io::IO, obj, indent::Int=0, name=nothing)
+  pre, post = obj isa Chain{<:AbstractVector} ? ("[", "]") : ("", "")
   children = trainable(obj)
   if all(_show_leaflike, children)
     _layer_show(io, obj, indent, name)
   else
-    println(io, " "^indent, isnothing(name) ? "" : "$name = ", nameof(typeof(obj)), "(")
+    println(io, " "^indent, isnothing(name) ? "" : "$name = ", nameof(typeof(obj)), "(", pre)
     if obj isa Chain{<:NamedTuple} && children == getfield(obj, :layers)
       # then we insert names -- can this be done more generically? 
       for k in Base.keys(obj)
@@ -35,10 +36,10 @@ function _big_show(io::IO, obj, indent::Int=0, name=nothing)
       end
     end
     if indent == 0  # i.e. this is the outermost container
-      print(io, ")")
+      print(io, post, ")")
       _big_finale(io, obj)
     else
-      println(io, " "^indent, "),")
+      println(io, " "^indent, post, "),")
     end
   end
 end

test/layers/basic.jl

@@ -26,6 +26,8 @@ import Flux: activations
     @test m[1:2] == m
 
     @test_throws ArgumentError Chain(layers = Dense(10, 10), two = identity) # reserved name
+
+    @test_nowarn Chain([Dense(10, 5, σ), Dense(5, 2)])(randn(Float32, 10))  # vector of layers
   end
 
   @testset "Activations" begin
@@ -274,6 +276,10 @@ end
   m1 = Chain(Dense(3,4,tanh; bias=false), Dense(4,2))
   @test Zygote.hessian_dual(sum∘m1, [1,2,3]) ≈ Zygote.hessian_reverse(sum∘m1, [1,2,3])
 
+  m1v = Chain([m1[1], m1[2]])  # vector of layers
+  @test Zygote.hessian_dual(sum∘m1v, [1,2,3]) ≈ Zygote.hessian_dual(sum∘m1, [1,2,3])
+  @test_broken Zygote.hessian_dual(sum∘m1v, [1,2,3]) ≈ Zygote.hessian_reverse(sum∘m1v, [1,2,3])
+
   # NNlib's softmax gradient writes in-place
   m2 = Chain(Dense(3,4,tanh), Dense(4,2), softmax)
   @test_broken Zygote.hessian_dual(sum∘m2, [1,2,3]) ≈ Zygote.hessian_reverse(sum∘m2, [1,2,3])
@@ -284,3 +290,16 @@ end
   @test_broken Zygote.hessian_dual(sum∘m3, x3) ≈ Zygote.hessian_reverse(sum∘m3, x3)
 end
 
+@testset "gradients of Chain{Vector}" begin
+  m1 = Chain(Dense(3,4,tanh; bias=false), Dense(4,2))
+  m1v = Chain([m1[1], m1[2]])
+  @test sum(length, params(m1)) == sum(length, params(m1v))
+
+  x1 = randn(Float32,3,5)
+  @test m1(x1) ≈ m1v(x1)
+  y1 = rand(Bool,2,5)
+  g1 = gradient(() -> Flux.Losses.logitcrossentropy(m1(x1), y1), params(m1))
+  g1v = gradient(() -> Flux.Losses.logitcrossentropy(m1v(x1), y1), params(m1v))
+  @test g1[m1[1].weight] ≈ g1v[m1v[1].weight]
+  @test g1[m1[2].bias] ≈ g1v[m1v[2].bias]
+end