Allow gradients in fmap

Project.toml

@@ -8,6 +8,7 @@ julia = "1"
 
 [extras]
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
+Zygote = "e88e6eb3-aa80-5325-afca-941959d7151f"
 
 [targets]
-test = ["Test"]
+test = ["Test", "Zygote"]

src/functor.jl

@@ -75,10 +75,29 @@ Equivalent to `functor(x)[1]`.
 """
 children(x) = functor(x)[1]
 
+function functor_tuple(f, x::Tuple, dx::Tuple)
+  map(x, dx) do x, x̄
+    _default_walk(f, x, x̄)
+  end
+end
+functor_tuple(f, x, dx) = f(x, dx)
+functor_tuple(f, x, ::Nothing) = x
+
+# @functor Chain
+# Chain -> func = (layers = (Dense,Dense),), gs -> (layers...)
+function _default_walk(f, x, dx)
+  func, re = functor(x)
+  map(func, dx) do x, x̄
+    # functor_tuple(f, x, x̄)
+    f(x, x̄)
+  end |> re
+end
+
 function _default_walk(f, x)
   func, re = functor(x)
   re(map(f, func))
 end
+_default_walk(f, ::Nothing, ::Nothing) = nothing
 
 """
     fmap(f, x; exclude = isleaf, walk = Functors._default_walk)
@@ -205,3 +224,11 @@ function fcollect(x; output = [], cache = Base.IdSet(), exclude = v -> false)
     end
     return output
 end
+
+# Allow gradients and other constructs that match the structure of the functor
+# to allow for `map` style computations and return a modified version of the struct.
+# This way we can use `fmap` to update the params with their gradients
+function fmap(f, x, dx...; cache = IdDict())
+  haskey(cache, x) && return cache[x]
+  cache[x] = isleaf(x) ? f(x, dx...) : _default_walk((x...) -> fmap(f, x..., cache = cache), x, dx...)
+end

test/runtests.jl

@@ -1,8 +1,8 @@
 using Functors, Test
+using Zygote
 
 @testset "Functors.jl" begin
 
-include("basics.jl")
-include("base.jl")
-
+  include("basics.jl")
+  include("update.jl")
 end

test/update.jl

@@ -0,0 +1,23 @@
+@testset "Generalized fmap over equivalent functors" begin
+  struct M{F,T,S}
+    σ::F
+    W::T
+    b::S
+  end
+
+  @functor M
+
+  (m::M)(x) = m.σ.(m.W * x .+ m.b)
+
+  m = M(identity, ones(Float32, 3, 4), zeros(Float32, 3))
+  x = ones(Float32, 4, 2)
+  m̄, _ = gradient((m,x) -> sum(m(x)), m, x)
+  m̂ = Functors.fmap(m, m̄) do x, y
+    isnothing(x) && return y
+    isnothing(y) && return x
+    x .- 0.1f0 .* y
+  end
+
+  @test m̂.W ≈ fill(0.8f0, size(m.W))
+  @test m̂.b ≈ fill(-0.2f0, size(m.b))
+end