Introduction

Usage

Say we have a deeply nested struct:

julia> using StaticArrays;

julia> struct Person
           name::Symbol
           age::Int
       end;

julia> struct SpaceShip
           captain::Person
           velocity::SVector{3, Float64}
           position::SVector{3, Float64}
       end;

julia> s = SpaceShip(Person(:julia, 2009), [0.0, 0.0, 0.0], [0.0, 0.0, 0.0])
SpaceShip(Person(:julia, 2009), [0.0, 0.0, 0.0], [0.0, 0.0, 0.0])

Lets update the captains name:

julia> s.captain.name = :JULIA
ERROR: type Person is immutable

It's a bit cryptic but what it means that Julia tried very hard to set the field but gave it up since the struct is immutable. So we have to do:

julia> SpaceShip(Person(:JULIA, s.captain.age), s.velocity, s.position)
SpaceShip(Person(:JULIA, 2009), [0.0, 0.0, 0.0], [0.0, 0.0, 0.0])

This is messy and things get worse, if the structs are bigger. Setfields to the rescue!

julia> using Setfield

julia> s = @set s.captain.name = :JULIA
SpaceShip(Person(:JULIA, 2009), [0.0, 0.0, 0.0], [0.0, 0.0, 0.0])

julia> s = @set s.velocity[1] += 999999
SpaceShip(Person(:JULIA, 2009), [999999.0, 0.0, 0.0], [0.0, 0.0, 0.0])

julia> s = @set s.velocity[1] += 1000001
SpaceShip(Person(:JULIA, 2009), [2.0e6, 0.0, 0.0], [0.0, 0.0, 0.0])

julia> @set s.position[2] = 20
SpaceShip(Person(:JULIA, 2009), [2.0e6, 0.0, 0.0], [0.0, 20.0, 0.0])

Under the hood

Under the hood this package implements a simple lens api. This api may be useful in its own rite and works as follows:

julia> using Setfield

julia> l = @lens _.a.b
(@lens _.a.b)

julia> struct AB;a;b;end

julia> obj = AB(AB(1,2),3)
AB(AB(1, 2), 3)

julia> set(obj, l, 42)
AB(AB(1, 42), 3)

julia> obj
AB(AB(1, 2), 3)

julia> get(obj, l)
2

julia> modify(x->10x, obj, l)
AB(AB(1, 20), 3)

Now the @set macro simply provides sugar for creating a lens and applying it. For instance

@set obj.a.b = 42

expands roughly to

l = @lens _.a.b
set(obj, l, 42)