FlexibleFunctors.jl allows you to convert struct fields to a flat vector representation, and it also provides a function to transform similar vectors back into structs. However, FlexibleFunctors.jl also provides a parameters(x) function which can be extended to dynamically determine which fields make it into the vector and which fields are fixed.


using FlexibleFunctors, Zygote
import FlexibleFunctors: parameters

struct Model
parameters(m::Model) = m.params

struct Square
parameters(s::Square) = s.params
area(s::Square) = s.w^2

s1 = Square(1.0, (:w,))
s2 = Square(2.0, ())
s3 = Square(3.0, (:w,))
m1 = Model([s1, s2, s3], (:elements,))
m2 = Model([s1, s2, s3], ())

p1, re1 = destructure(m1)
p2, re2 = destructure(m2)

julia> println(p1)
# [1.0, 3.0]

julia> println(p2)
# Any[]

# Replace first and third widths with vector values
julia> mapreduce((s)->area(s), +, re1([3.0, 4.0]).elements)
# 29

# Uses original widths since `m2` has no parameters
julia> mapreduce((s)->area(s), +, re2([3.0, 4.0]).elements)
# 14

# Map vector of parameters to an object for optimization
grad = Zygote.gradient((x) -> mapreduce(area, +, x.elements), re1([3.0, 4.0]));

julia> getfield.(grad[1].elements, :w)
# [6.0, 4.0, 8.0]


FlexibleFunctors.jl adds the ability to specify individual fields of a type as parameters. These parameters indicate fields which can be retrieved to obtain a flat vector representation of the struct parameters through destructure. destructure also returns a function, re, which reconstructs an instance of the original type. re operates as a function of the flat vector representation, but the reconstructed values of parameter fields are drawn from this vector while non-parameter fields are left unchanged.

Parameters are determined recursively. If a tagged field contains a struct with parameters of its own, those parameters are also included in the resulting vector and reconstruction.

ffunctor takes a struct instance with a parameters method and returns a NamedTuple representation of the parameters. ffunctor also returns a re function for reconstructing the struct from the NamedTuple representation.

In either case, you must provide a FlexibleFunctors.parameters(::YourType) method which return the parameters of YourType in a Tuple of Symbols. This is accomplished by importing FlexibleFunctors.jl and adding additional methods directly to parameters.

What's Flexible about FlexibleFunctors?

The functionality provided by FlexibleFunctors.jl is similar to Functors.jl. Both projects annotate types with special fields to convert in between flat vector representations of structs and a reconstructed instance. However, Functors.jl stores this information at the type-level for all instances of a type, forcing all instances to use identical fields as (what we call) parameters.

FlexibleFunctors.jl can store these parameter fields at the instance-level. For example, instead of hard-coding the returned parameters, we could add a parameters field to a struct, and then specify these parameters at run-time as a tuple of Symbols. In this way, we can use the same types but specialize which fields are actually parameterized and liable to be changed during a reconstruction.

If parameter fields are stored within a struct instance, re-parameterizing the type instance is possible. For this, we recommend Setfield.jl and the @set macro which can easily update nested fields.

Further, note that FlexibleFunctors.jl can reproduce similar behavior to Functors.jl. If parameters(m::M) = (:some, :fixed, :tuple), then all instances of M will have the same parameters.