Reconstruct with all fieldnames

This ensures that important, non-fmappable properties with non-default values are preserved.

Author: ToucheSir

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