# Implementing interface on custom types

**URL:** <https://discourse.julialang.org/t/implementing-interface-on-custom-types/66250>\
**Category:** General Usage\
**Tags:** question, interface\
**Created:** [August 12, 2021, 8:03am UTC](https://discourse.julialang.org/t/implementing-interface-on-custom-types/66250 "2021-08-12T08:03:27Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Daneel](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/daneel/32/175_2.png) [@Daneel](https://discourse.julialang.org/u/Daneel)\
**Post date:** [August 12, 2021, 8:03am UTC](https://discourse.julialang.org/t/implementing-interface-on-custom-types/66250/1 "2021-08-12T08:03:31Z")

</div>

Is it possible to implement an interface on a simple custom type so that many common functions will work on it? I’m trying to improve my knowledge of Julia. Let’s say we have the following simple custom type.

```julia
mutable struct interfaceTest
	value::Int
	x::Bool
	theAnswer::Bool
	
	function interfaceTest(val::Float64)
		interfaceTest(Int(val))
	end
	
	function interfaceTest(val::Int)
		value = val
		x = val > 10
		theAnswer = val == 42
		
		new(value,x,theAnswer)
	end
end

x = [3,5,48,30,2,2,2,42,21,15,12]
y = interfaceTest.(x)

```

`y.value` is the important factor, the rest are derived values. I would like to use simple functions like `mean`, `unique`, `minimum`, and `maximum`. It’s simple enough to extend each function but it would be _very_ practical to have one way of doing it all at once.

Using the [interfaces](https://docs.julialang.org/en/v1/manual/interfaces/) section of the manual I tried this:

```julia
function Base.iterate(test::interfaceTest, state=1)
	state > 1 ? nothing : (test.value, state+1)
end

```

Testing `unique` resulted in the same result, even though there are duplicate values in the original data set, it returned every of the `interfaceTest` values as unique.

---

<div class="post-metadata">

**Author:** ![sostock](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sostock/32/5546_2.png) [@sostock](https://discourse.julialang.org/u/sostock)\
**Post date:** [August 12, 2021, 9:38am UTC](https://discourse.julialang.org/t/implementing-interface-on-custom-types/66250/2 "2021-08-12T09:38:31Z")

</div>

> [@Daneel](#):
>
> Testing `unique` resulted in the same result, even though there are duplicate values in the original data set, it returned every of the `interfaceTest` values as unique.

That is because these “duplicate values” are actually unique, i.e., they are not equal to each other:

```julia
julia> a = interfaceTest(2)
interfaceTest(2, false, false)

julia> b = interfaceTest(2)
interfaceTest(2, false, false)

julia> a == b
false

```

This is because `==` falls back to `===` which returns false because they have different addresses in memory. You need to either implement a custom `==` method or turn `interfaceTest` into a non-mutable `struct` to fix this.

In order to get the other functions (like `maximum`) to work, you would need to implement `max`, `+`, etc. Iteration does not help here.

---

<div class="post-metadata">

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [August 12, 2021, 11:28am UTC](https://discourse.julialang.org/t/implementing-interface-on-custom-types/66250/3 "2021-08-12T11:28:40Z")

</div>

Maybe something like this is what you want?

```julia
julia> struct A
          x::Float64
       end

julia> using Statistics

julia> Statistics.mean(avec::AbstractVector{A}) = mean(a.x for a in avec)

julia> avec = A.(x)
3-element Vector{A}:
 A(1.0)
 A(2.0)
 A(3.0)

julia> mean(avec)
2.0

```

To get mean to work without defining it explicitly, you would need to define the operations:

```julia
julia> import Base:+,-,/

julia> +(x::A,y::A) = x.x + y.x
+ (generic function with 191 methods)

julia> /(x::A,y) = x.x/y
/ (generic function with 108 methods)

julia> +(x,y::A) = x + y.x
+ (generic function with 192 methods)

julia> mean(avec)
2.0

```

---

<div class="post-metadata">

**Author:** ![Daneel](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/daneel/32/175_2.png) [@Daneel](https://discourse.julialang.org/u/Daneel)\
**Post date:** [August 12, 2021, 11:43am UTC](https://discourse.julialang.org/t/implementing-interface-on-custom-types/66250/4 "2021-08-12T11:43:00Z")

</div>

Too bad it’s not as easy as I’d hoped. The difference with `mutable` is good to know.

@lmiq That is what I had meant about extending each function but it is helpful to see the actual implementations.