Accessors

DocStable DocDev CI

The goal of Accessors.jl is to make updating immutable data simple. It is the successor of Setfield.jl.

Usage

Say you have some immutable data structure, such as a NamedTuple:

julia> nt = (a=1, b=2)
(a = 1, b = 2)

If you try something like nt.b=3, it will throw an error. But using Accessors, we can change it anyways:

julia> using Accessors

julia> @set nt.b=3
(a = 1, b = 3)

Note that this only returns an updated copy of nt, and does not overwrite or mutate the value bound to nt:

julia> nt
(a = 1, b = 2)

To overwrite the old definition, we can rebind nt to the new version:

julia> nt = @set nt.b=3
(a = 1, b = 3)

julia> nt
(a = 1, b = 3)

As this is a common use case, the convenience macro @reset rebinds the variable (nt) to the updated version:

julia> @reset nt.b=4
(a = 1, b = 4)

julia> nt
(a = 1, b = 4)

For more detail, see this tutorial and/or watch this video:

JuliaCon2020 Changing the immutable

Featured extensions

  • AccessorsExtra.jl [docs] introduces additional optics and related functions, that are considered too experimental for inclusion in Accessors. For Julia 1.8 and older, AccessorsExtra also provides integrations with other packages; for Julia 1.9+, these are mostly included in Accessors itself.