# Revise and const values changed during a session?

**URL:** <https://discourse.julialang.org/t/revise-and-const-values-changed-during-a-session/64372>\
**Category:** General Usage\
**Created:** [July 9, 2021, 6:18pm UTC](https://discourse.julialang.org/t/revise-and-const-values-changed-during-a-session/64372 "2021-07-09T18:18:22Z")\
**Posts on this page:** 1\
**Showing post:** 3

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**Author:** ![sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@sapo](https://discourse.julialang.org/u/sapo)\
**Post date:** [July 12, 2021, 8:38am UTC](https://discourse.julialang.org/t/revise-and-const-values-changed-during-a-session/64372/3 "2021-07-12T08:38:03Z")

</div>

Thank you so much! Today I have prepared a little test comparing your solution and another idea I had involving mutable structs. Your solution is by far the best (even better the not-working one).

Here the code and tests:

```julia
# A.jl
module A

# METHOD 1
# the following works but are not re-evaluated by Revise
const a = 5
b = 5

# METHOD 2
# functions are re-evaluated by Revise when they are changed
const c() = 5
d() = 5

# METHOD 3
mutable struct S
    vars::NamedTuple
    function S()
        # the following ensures that S cannot be reinstantiated without explicit
        # usage of `eval`
        isdefined(S, :s) && error("Cannot instantiate S twice")
        return new()
    end
end

# `const` ensures that the reference to the object will always be the same
const s = S()

# an easier way to get the settings
function getS()
    return s.vars
end

# defining fields in a function allows Revise to re-evaluate it when settings
# are changed
function init()
    s.vars = (a = 5, b = 5)
end

end

```

```julia
# B.jl
module B
using Main.A: a, b, c, d, getS
function test1()
    r = 0
    for i in 1:1000
        r += a + b
    end
    return r
end
function test2()
    r = 0
    for i in 1:1000
        r += c() + d()
    end
    return r
end
function test3()
    s = getS()
    r = 0
    for i in 1:1000
        r += s.a + s.b
    end
    return r
end
end

```

```julia
# C.jl
module C
using BenchmarkTools
using Main.B: test1, test2, test3
using Main.A: init

function main()
    init()
    println(@btime test1())
    println(@btime test2())
    println(@btime test3())
end
end

```

```julia
# D.jl
using Revise

includet("A.jl")
includet("B.jl")
includet("C.jl")

```

And here the tests:

```julia
julia> include("D.jl");

julia> C.main()
k 30.400 μs (958 allocations: 14.97 KiB)
12000
  1.492 ns (0 allocations: 0 bytes)
12000
  62.388 μs (958 allocations: 14.97 KiB)
12000

julia> C.main() # re-run after having changed the settings
  30.289 μs (958 allocations: 14.97 KiB)
12000
  1.492 ns (0 allocations: 0 bytes)
10000
  62.629 μs (949 allocations: 14.83 KiB)
10000

```

As you see, method 1 doesn’t work on re-evaluation, method 3 works but is 2x slower, while method 2 works and is 20.000x faster

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_[View the full topic](https://discourse.julialang.org/t/revise-and-const-values-changed-during-a-session/64372)._