add test for generic f-map

Author: DhairyaLGandhi

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