# How to change a public interface

**URL:** <https://discourse.julialang.org/t/how-to-change-a-public-interface/97958>\
**Category:** General Usage\
**Created:** [April 26, 2023, 5:30pm UTC](https://discourse.julialang.org/t/how-to-change-a-public-interface/97958 "2023-04-26T17:30:44Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Lilith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lilith/32/27492_2.png) [@Lilith](https://discourse.julialang.org/u/Lilith)\
**Post date:** [April 26, 2023, 5:30pm UTC](https://discourse.julialang.org/t/how-to-change-a-public-interface/97958/1 "2023-04-26T17:30:44Z")

</div>

Let’s say I have a package that defines an abstract type with an interface: if you define `successor_is_prime`, then my module will define `predecessor_is_prime`.

```julia
module Arithmetic
    function successor_is_prime end
    predecessor_is_prime(x::MyAbstractNumber) = successor_is_prime(x-2)
end

```

Now, I realize that I would rather have a new function `is_prime`, and define all the other functions in terms of it so the new interface is: if you define `is_prime` then my module will define `predecessor_is_prime` and `successor_is_prime`:

```julia
module Arithmetic
    function is_prime end
    predecessor_is_prime(x::MyAbstractNumber) = is_prime(x-1)
    sucessor_is_prime(x::MyAbstractNumber) = is_prime(x+1)
end

```

Is there a way to do this without breaking changes and without creating a dispatch loop that causes a StackOverflow error when none of the methods are defined for a concrete subtype?

(The real-world problem that motivates this is adding `modify!` and defining all other `AbstractDict` operations in terms of it, making the AbstractDict interface: implement `modify!` (and optionally iterate) and get everything else for free. Related to [https://github.com/JuliaLang/julia/pull/33758](https://github.com/JuliaLang/julia/pull/33758))

---

<div class="post-metadata">

**Author:** ![cjdoris](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cjdoris/32/213133_2.png) [@cjdoris](https://discourse.julialang.org/u/cjdoris)\
**Post date:** [April 26, 2023, 9:49pm UTC](https://discourse.julialang.org/t/how-to-change-a-public-interface/97958/2 "2023-04-26T21:49:05Z")

</div>

One way is to also add a trait specifying which interface is implemented, and assume the old one by default. For example:

```julia
supports_is_prime(x) = false
supports_is_prime(x::MyAbstractNumber) = true
predecessor_is_prime(x) = supports_is_prime(x) ? is_prime(x-1) : throw(MethodError(...))
successor_is_prime(x) = supports_is_prime(x) ? is_prime(x+1) : predecessor_is_prime(x+2)

```