Freeze.jl

A very simple wrapper structure to make objects fully recursively immutable. It disallows setproperty!, push!, append!, setindex! and other mutating operations, but implements getproperty, getindex and iterate, all of which return Frozen for non-bitstype data. Example:

julia> using Freeze

julia> struct MyData
        xs::Vector{Int}
        ys::Vector{Int}
    end
julia> data = freeze(MyData([1, 2, 3], [4, 5, 6]))
Frozen{MyData}(MyData([1, 2, 3], [4, 5, 6]))

julia> data.xs[1]
1

julia> data.xs[1] = 10
ERROR: Cannot change frozen object
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] _err()
   @ Freeze ~/.../Freeze.jl/src/Freeze.jl:11
 [3] setindex!(::Frozen{Vector{Int64}}, ::Int64, ::Int64)
   @ Freeze ~/.../Freeze.jl/src/Freeze.jl:12
 [4] top-level scope
   @ REPL[4]:1

julia> push!(data.xs, 8)
ERROR: Cannot change frozen object

julia> dict = freeze(Dict{String, MyData}(
    "First" => MyData([1, 2, 3], [4, 5, 6]),
    "Second" => MyData([7, 8, 9], [10, 11, 12]))
    )
Frozen{Dict{String, MyData}}(Dict{String, MyData}("Second" => MyData([7, 8, 9], [10, 11, 12]), "First" => MyData([1, 2, 3], [4, 5, 6])))

julia> for (k, v) in dict
           @show k, v
           @show v.ys[1]
           # iterator over Frozen yeilds Frozen, so the next line will throw if uncommented
           # v.xs[2] = 10
       end
(k, v) = ("Second", Frozen{MyData}(MyData([7, 8, 9], [10, 11, 12])))
v.ys[1] = 10
(k, v) = ("First", Frozen{MyData}(MyData([1, 2, 3], [4, 5, 6])))
v.ys[1] = 4

Interface

Frozen(x) - construct Frozen object directly from passed value
freeze(x) - construct Frozen object from a copy of passed value. To customize copying override clone. For Frozen arguments, returns the argument.
unfreeze(x) - get a mutable copy of stored data.