Reconstruct with all fieldnames

README.md

@@ -49,6 +49,25 @@ Foo(1.0, [1.0, 2.0, 3.0])
 
 `functor` returns the parts of the object that can be inspected, as well as a `re` function that takes those values and restructures them back into an object of the original type.
 
-For a discussion regarding implementing functors for which only a subset of the fields are "seen" by `functor`, see [here](https://github.com/FluxML/Functors.jl/issues/3#issuecomment-626747663).
+To include only certain fields, pass a tuple of field names to `@functor`:
+
+```julia
+julia> struct Baz
+         x
+         y
+       end
+
+julia> @functor Baz (x,)
+
+julia> model = Baz(1, 2)
+Baz(1, 2)
+
+julia> fmap(float, model)
+Baz(1.0, 2)
+```
+
+Any field not in the list will not be returned by `functor` and passed through as-is during reconstruction. This is done by invoking the default constructor, so structs that define custom inner constructors are expected to provide one that acts like the default.
+
+It is also possible to implement `functor` by hand when greater flexibility is required. See [here](https://github.com/FluxML/Functors.jl/issues/3) for an example.
 
 For a discussion regarding the need for a `cache` in the implementation of `fmap`, see [here](https://github.com/FluxML/Functors.jl/issues/2).

src/functor.jl

@@ -8,8 +8,14 @@ functor(::Type{<:AbstractArray}, x) = x, y -> y
 functor(::Type{<:AbstractArray{<:Number}}, x) = (), _ -> x
 
 function makefunctor(m::Module, T, fs = fieldnames(T))
+  yᵢ = 0
+  escargs = map(fieldnames(T)) do f
+    f in fs ? :(y[$(yᵢ += 1)]) : :(x.$f)
+  end
+  escfs = [:($f=x.$f) for f in fs]
+
   @eval m begin
-    $Functors.functor(::Type{<:$T}, x) = ($([:($f=x.$f) for f in fs]...),), y -> $T(y...)
+    $Functors.functor(::Type{<:$T}, x) = ($(escfs...),), y -> $T($(escargs...))
   end
 end
 

test/basics.jl

@@ -1,21 +1,38 @@
 using Functors, Test
 
-struct Foo
-  x
-  y
-end
+@testset "Nested" begin
+  struct Foo
+    x
+    y
+  end
 
-@functor Foo
+  @functor Foo
 
-struct Bar
-  x
-end
+  struct Bar
+    x
+  end
 
-@functor Bar
+  @functor Bar
 
-model = Bar(Foo(1, [1, 2, 3]))
+  model = Bar(Foo(1, [1, 2, 3]))
 
-model′ = fmap(float, model)
+  model′ = fmap(float, model)
 
-@test model.x.y == model′.x.y
-@test model′.x.y isa Vector{Float64}
+  @test model.x.y == model′.x.y
+  @test model′.x.y isa Vector{Float64}
+end
+
+@testset "Property list" begin
+  struct Baz
+    x
+    y
+    z
+  end
+
+  @functor Baz (y,)
+
+  model = Baz(1, 2, 3)
+  model′ = fmap(x -> 2x, model)
+
+  @test (model′.x, model′.y, model′.z) == (1, 4, 3)
+end