Fallback to_vec intended for structs

Project.toml

@@ -1,6 +1,6 @@
 name = "FiniteDifferences"
 uuid = "26cc04aa-876d-5657-8c51-4c34ba976000"
-version = "0.12.0"
+version = "0.12.1"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"

src/to_vec.jl

@@ -21,6 +21,25 @@ end
 # Base case -- if x is already a Vector{<:Real} there's no conversion necessary.
 to_vec(x::Vector{<:Real}) = (x, identity)
 
+# Fallback method for `to_vec`. Won't always do what you wanted, but should be fine a decent
+# chunk of the time.
+function to_vec(x::T) where {T}
+    Base.isstructtype(T) || throw(error("Expected a struct type"))
+    isempty(fieldnames(T)) && return (Bool[], _ -> x) # Singleton types
+
+    val_vecs_and_backs = map(name -> to_vec(getfield(x, name)), fieldnames(T))
+    vals = first.(val_vecs_and_backs)
+    backs = last.(val_vecs_and_backs)
+
+    v, vals_from_vec = to_vec(vals)
+    function structtype_from_vec(v::Vector{<:Real})
+        val_vecs = vals_from_vec(v)
+        vals = map((b, v) -> b(v), backs, val_vecs)
+        return T(vals...)
+    end
+    return v, structtype_from_vec
+end
+
 function to_vec(x::AbstractVector)
     x_vecs_and_backs = map(to_vec, x)
     x_vecs, backs = first.(x_vecs_and_backs), last.(x_vecs_and_backs)
@@ -169,7 +188,6 @@ function FiniteDifferences.to_vec(x::Composite{P}) where{P}
     return x_vec, Composite_from_vec
 end
 
-
 function FiniteDifferences.to_vec(x::AbstractZero)
     function AbstractZero_from_vec(x_vec::Vector)
         return x

test/to_vec.jl

@@ -25,6 +25,12 @@ struct ThreeFields
     c
 end
 
+# For testing nested fallback for structs
+struct Singleton end
+struct Nested
+    x::ThreeFields
+    y::Singleton
+end
 
 Base.size(x::FillVector) = (x.len,)
 Base.getindex(x::FillVector, n::Int) = x.x
@@ -168,4 +174,9 @@ end
         x_vec, from_vec = to_vec(x)
         @test_throws MethodError from_vec(randn(10))
     end
+
+    @testset "fallback" begin
+        nested = Nested(ThreeFields(1.0, 2.0, "Three"), Singleton())
+        test_to_vec(nested; check_inferred=false) # map
+    end
 end