ANN: Setfield.jl Yet another package for working with immutables

General Usage
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44

I started liking lens-based approach! Now that lens can change type parameters, we have “differentiable lens”!

using ForwardDiff
using Parameters: @with_kw, @unpack
using Setfield: @lens, Lens, set, get
import Setfield

@with_kw struct Coordinate{X, Y, Z}
    x::X = 1.0
    y::Y = 1.0
    z::Z = 1.0
end

# Since set called via ForwardDiff would specify a dual number, we
# have to ignore type parameters:
Setfield.constructor_of(::Type{<: Coordinate}) = Coordinate

function f(c::Coordinate)
    @unpack x, y, z = c
    return x^2 + y^2 + z^2
end

derivative(f, at, wrt::Lens) =
    ForwardDiff.derivative(
        (x) -> f(set(wrt, at, x)),
        get(wrt, at))

@assert derivative(f, Coordinate(x=1.0), @lens _.x) ≈ 2
@assert derivative(f, Coordinate(x=2.0), @lens _.x) ≈ 4

This is actually super useful for building something like bifurcation analysis tool where you need a generic way to let user specify a few parameters in a differentiable manner.

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