FlexibleFunctors.jl provides a utility for destructuring, restructuring, and mapping operations over a flexible set of parameters for any type.
Parameter tagging is flexible in its implementation, permitting either global or uniqueperinstance parameters. Parameter extraction is recursive, extracting tagged parameters from within nested tagged fields to a flat vector representation along with a function to reconstruct the original struct. These flat representations can be modified and then reconstructed to alter the underlying parameter values.
Our syntax and inspiration draw from Functors.jl
. Our implementation differs in that parameters mapped to a flat vector or NamedTuple
representation can vary betweens instances of a struct.
Usage
The core usage of FlexibleFunctors.jl
lies in destructure
. Given an object, destructure
returns a flat vector representation of the tagged parameters from that instance and a function to reconstruct that object. This reconstruction function replaces all parameters with those from its input and leaves other fields untouched.
FlexibleFunctors.jl
's flexibility comes from a small function to extend with methods for your types, parameters(m::MyType)
. The role of this function is to return parameter fields as a Tuple
of Symbol
s from the given instance m
. These symbols can be constant (reproducing behavior akin to Functors.jl
), or they can rely on the information stored within m
itself. A params
fields within a struct can be used to store parameters on and instancebyinstance case, or parameters
can inspect the types of m
's fields to determine parameters, given rise to ModelParameters.jl
like usage.
Functions can be mapped over all of the parameters using the fieldmap
export. The syntax is similar to traditional map
: fieldmap(f, x)
maps f
over every parameter within x
and updates these values in the returned struct.
In tandem with SetField.jl, we can update a given struct’s parameters inplace. If parameters are annotated within a field of a struct, SetField.jl
can update these parameters without needing to manually reconstruct a complicated model.
Example
As an example, we consider a problem of summing the areas of squares in a collection. Some squares have their w
idth as a parameter, but one doesn’t. These Square
s are compiled into Model
s, which themselves can tag their constituent elements
as containing parameters. We are able to re
structure the Square
s nested within the Model
, and use these resulting structs as normal.
using FlexibleFunctors, Zygote
import FlexibleFunctors: parameters
struct Model
elements
params
end
parameters(m::Model) = m.params
struct Square
w
params
end
parameters(s::Square) = s.params
area(s::Square) = s.w^2
# Squares 1 and 3 tag w as a parameter, 2 has no parameters
s1 = Square(1.0, (:w,))
s2 = Square(2.0, ())
s3 = Square(3.0, (:w,))
# Model 1 tags elements as containing parameters, Model 2 has none
m1 = Model([s1, s2, s3], (:elements,))
m2 = Model([s1, s2, s3], ())
p1, re1 = destructure(m1)
p2, re2 = destructure(m2)
julia> p1
# [1.0, 3.0]
julia> p2
# Any[]
# Replace first and third widths with new values
julia> mapreduce((s)>area(s), +, re1([3.0, 4.0]).elements)
# 29
# Uses original widths since `m2` has no parameters indicated
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]
Caveats
For FlexibleFunctors.jl
, reconstruction is not guaranteed to be typestable. If the values in a reconstruction vector differ in type from the original values, no error will necessarily be thrown if they are compatible with the underlying type. Care should be taken with respect to replacing Float
s with Int
s and similar cases. Comparable packages below treat this case more deliberately than we have.
We currently don’t perform any checks on the argument to a re
construction function. If your reconstruciton vector is longer than the number of parameters being reconstructed, the first elements of the vector will be used for their respective parameters and later elements will be ignored.
Related Work

Functors.jl served as primary inspiration for this package. We wanted the ability to easily instantiate complex structures from a flat vector representation, but we needed individual instances of any given struct to have different parameter fields. As it stands,
Functors.jl
is global across all instances of any given struct. 
ParameterHandling.jl focuses on constraining the parameters and manually extending
flatten
methods.FlexibleFunctors.jl
will automatically extract parameters once they’ve been annotated within a struct, andparameters
methods should easily customize to your suit your needs. 
ModelingParameters.jl is a great package which implements a very nice tool where
Param
types denote parameters within a struct. TheseParam
s are used to tag parameters, as opposed to our method where an external method operates on a struct to determine parameters with customizable behavior.