Allow Parallel(+, f)(x, y, z) to work like broadcasting, and enable Chain(identity, Parallel(+, f))(x, y, z)

src/layers/basic.jl

@@ -28,6 +28,20 @@ julia> m2(x) == (m2[:dec] ∘ m2[:enc])(x)
 true
 ```
 
+A chain may be called with multiple arguments, which is equivalent to calling it
+with one tuple of these arguments. Such a tuple is understood by [`Parallel`](@ref)
+to mean the same as several arguments:
+
+```jldoctest
+julia> Chain(println, println)(1, 2, 3)  # three arguments become a tuple
+(1, 2, 3)
+nothing
+
+julia> Chain(x->@show(x), Parallel(+, inv, abs2))(4, 5)  # returns 1/4 + 5^2
+x = (4, 5)
+25.25
+```
+
 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!
@@ -46,9 +60,10 @@ end
 @forward Chain.layers Base.getindex, Base.length, Base.first, Base.last,
   Base.iterate, Base.lastindex, Base.keys, Base.firstindex
 
-@layer :expand Chain  # the + opts-in to container-style pretty-printing
+@layer :expand Chain  # the option :expand opts-in to container-style pretty-printing
 
 (c::Chain)(x) = _applychain(c.layers, x)
+(c::Chain)(x, ys...) = _applychain(c.layers, (x, ys...))
 
 @generated function _applychain(layers::Tuple{Vararg{Any,N}}, x) where {N}
   symbols = vcat(:x, [gensym() for _ in 1:N])
@@ -68,6 +83,7 @@ end
 Base.getindex(c::Chain, i::AbstractArray) = Chain(c.layers[i])
 Base.getindex(c::Chain{<:NamedTuple}, i::AbstractArray) =
   Chain(NamedTuple{keys(c)[i]}(Tuple(c.layers)[i]))
+
 function Base.show(io::IO, c::Chain)
   print(io, "Chain(")
   _show_layers(io, c.layers)
@@ -475,8 +491,11 @@ end
 Create a layer which passes an input array to each path in
 `layers`, before reducing the output with `connection`.
 
-Called with one input `x`, this is equivalent to `connection([l(x) for l in layers]...)`.
-If called with multiple inputs, one is passed to each layer, thus `Parallel(+, f, g)(x, y) = f(x) + g(y)`.
+Obeys the similar rules to broadcasting:
+* Called with one input `x`, this is equivalent to `connection([l(x) for l in layers]...)`.
+* With multiple `inputs` and just one layer, it is instead `connection([layer(x) for x in inputs]...)`.
+* With multiple inputs and multiple layers, one input is passed to each layer,
+  thus `Parallel(+, f, g)(x, y) = f(x) + g(y)`.
 
 Like [`Chain`](@ref), its sub-layers may be given names using the keyword constructor.
 These can be accessed by indexing: `m[1] == m[:name]` is the first layer.
@@ -486,6 +505,25 @@ and [`Maxout`](@ref) which reduces by broadcasting `max`.
 
 # Examples
 
+```jldoctest
+julia> p = Parallel(+, abs2, sqrt);
+
+julia> p(3, 4)  # == 3^2 + √4, two functions two inputs
+11.0
+
+julia> p((3, 4))  # tuple is always splatted
+11.0
+
+julia> p(4)  # == 4^2 + √4, one input used twice
+18.0
+
+julia> Parallel(hcat, inv)(1, 2, 4)  # one function three inputs
+1×3 Matrix{Float64}:
+ 1.0  0.5  0.25
+```
+
+With Flux layers:
+
 ```jldoctest
 julia> model = Chain(Dense(3 => 5),
                      Parallel(vcat, Dense(5 => 4), Chain(Dense(5 => 7), Dense(7 => 4))),
@@ -516,35 +554,47 @@ struct Parallel{F, T<:Union{Tuple, NamedTuple}}
   layers::T
 end
 
+_ParallelONE{T} = Parallel{T, <:Union{Tuple{Any}, NamedTuple{<:Any, <:Tuple{Any}}}}
+
 Parallel(connection, layers...) = Parallel(connection, layers)
 function Parallel(connection; kw...)
   layers = NamedTuple(kw)
   if :layers in keys(layers) || :connection in keys(layers)
     throw(ArgumentError("a Parallel layer cannot have a named sub-layer called `connection` or `layers`"))
   end
-  isempty(layers) && return Parallel(connection, ())
   Parallel(connection, layers)
 end
+Parallel(connection, layers::Union{Tuple{}, @NamedTuple{}}) =
+    throw(ArgumentError("cannot construct a Parallel layer with no sub-layers"))
 
 @layer :expand Parallel
 
-(m::Parallel)(x) = m.connection(map(f -> f(x), Tuple(m.layers))...)
-(m::Parallel)(xs::Tuple) = m(xs...)
+(m::Parallel)(x) = m.connection(map(f -> f(x), Tuple(m.layers))...)  # one argument
 
 function _parallel_check(layers, xs)
   nl = length(layers)
-  nx = length(xs) 
+  @assert nl > 1  # dispatch handles nl==1 cases
+  nx = length(xs)
   if (nl != nx)
-    throw(ArgumentError(lazy"Parallel with $nl sub-layers can take one input or $nl inputs, but got $nx inputs"))
+    throw(ArgumentError(lazy"Parallel with $nl > 1 sub-layers can take one input or $nl inputs, but got $nx inputs"))
   end
 end
 ChainRulesCore.@non_differentiable _parallel_check(nl, nx)
 
-function (m::Parallel)(xs...)
+function (m::Parallel)(x, ys...)
+  xs = (x, ys...)
   _parallel_check(m.layers, xs)
-  m.connection(map(|>, xs, Tuple(m.layers))...)
+  m.connection(map(|>, xs, Tuple(m.layers))...)  # multiple arguments & multiple layers
 end
 
+(m::_ParallelONE)(x, ys...) =
+  m.connection(map(z -> only(m.layers)(z), (x, ys...))...)  # multiple arguments, one layer
+
+(m::Parallel)(xs::Tuple) = m(xs...)  # tuple is always splatted
+(m::_ParallelONE)(xs::Tuple) = m(xs...)  # solves an ambiguity
+
+(m::Parallel)() = throw(ArgumentError("Parallel layer cannot take 0 inputs"))
+
 Base.getindex(m::Parallel, i) = m.layers[i]
 Base.getindex(m::Parallel, i::AbstractVector) = Parallel(m.connection, m.layers[i])
 Base.getindex(m::Parallel{<:Any, <:NamedTuple}, i::AbstractVector) =

test/layers/basic.jl

@@ -35,6 +35,8 @@ using Flux: activations
     c = Chain(Dense(10, 5, σ), Dense(5, 2), Dense(2, 1, relu))
     @test c[1] == c[begin]
     @test c[3] == c[end]
+
+    @test Chain(identity)(1,2,3) == (1,2,3)  # multiple args become a tuple
   end
 
   @testset "Activations" begin
@@ -228,17 +230,20 @@ using Flux: activations
     end
 
     @testset "concat size" begin
-      input = randn(10, 2)
+      input = randn32(10, 2)
       @test size(Parallel((a, b) -> cat(a, b; dims=2), Dense(10, 10), identity)(input)) == (10, 4)
       @test size(Parallel(hcat, one = Dense(10, 10), two = identity)(input)) == (10, 4)
     end
 
     @testset "vararg input" begin
-      inputs = randn(10), randn(5), randn(4)
+      inputs = randn32(10), randn32(5), randn32(4)
       @test size(Parallel(+, Dense(10, 2), Dense(5, 2), Dense(4, 2))(inputs)) == (2,)
       @test size(Parallel(+; a = Dense(10, 2), b = Dense(5, 2), c = Dense(4, 2))(inputs)) == (2,)
       @test_throws ArgumentError Parallel(+, sin, cos)(1,2,3)  # wrong number of inputs
-      @test Parallel(+, sin, cos)(pi/2) ≈ 1
+      @test Parallel(+, sin, cos)(pi/2) ≈ 1  # one input, several layers
+      @test Parallel(/, abs)(3, -4) ≈ 3/4    # one layer, several inputs
+      @test Parallel(/, abs)((3, -4)) ≈ 3/4
+      @test Parallel(/; f=abs)(3, -4) ≈ 3/4
     end
 
     @testset "named access" begin
@@ -256,9 +261,13 @@ using Flux: activations
     end
 
     @testset "trivial cases" begin
-      @test Parallel(hcat) isa Parallel{typeof(hcat), Tuple{}}  # not a NamedTuple
-      @test Parallel(hcat)(1) == hcat()
-      @test Parallel(hcat, inv)(2) == hcat(1/2)  # still calls connection once.
+      # zero inputs, always an error
+      @test_throws ArgumentError Parallel(hcat)()
+      @test_throws ArgumentError Parallel(hcat, inv)()
+      @test_throws ArgumentError Parallel(hcat, inv, sqrt)()
+
+      # zero layers -- not useful... now made an error
+      @test_throws ArgumentError Parallel(hcat)
     end
 
     @testset "connection is called once" begin
@@ -270,6 +279,8 @@ using Flux: activations
       @test CNT[] == 2
       Parallel(f_cnt, sin)(1)
       @test CNT[] == 3
+      Parallel(f_cnt, sin)(1,2,3)
+      @test CNT[] == 4
     end
 
     # Ref https://github.com/FluxML/Flux.jl/issues/1673