# Make Base.unique work with user-defined types

**URL:** <https://discourse.julialang.org/t/make-base-unique-work-with-user-defined-types/85716>\
**Category:** General Usage\
**Created:** [August 13, 2022, 8:02pm UTC](https://discourse.julialang.org/t/make-base-unique-work-with-user-defined-types/85716 "2022-08-13T20:02:58Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Lewis\_Hein](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lewis_hein/32/23540_2.png) [@Lewis\_Hein](https://discourse.julialang.org/u/Lewis_Hein)\
**Post date:** [August 13, 2022, 8:02pm UTC](https://discourse.julialang.org/t/make-base-unique-work-with-user-defined-types/85716/1 "2022-08-13T20:02:58Z")

</div>

I am trying to make `Base.unique` work with an array of a custom type. A really trivial example might look like this

```julia
import Base.isequal
struct TestStruct
	x::String
end

Base.isequal(t1::TestStruct, t2::TestStruct) = length(t1.x) == length(t2.x)

#Should return just one element, instead returns exactly the input
unique([TestStruct("blah"), TestStruct("bluh")]) 

#Should return true, instead returns false
TestStruct("blah") == TestStruct("bluh")

```

I know `unique(f, itr)` exists, but it won’t work well for my actual problem. I want to be able to extend `Base.isequal` and have things like `unique` and `==` call it. How is this done?

---

<div class="post-metadata">

**Author:** ![digital\_carver](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/digital_carver/32/33818_2.png) [@digital\_carver](https://discourse.julialang.org/u/digital_carver)\
**Post date:** [August 13, 2022, 8:30pm UTC](https://discourse.julialang.org/t/make-base-unique-work-with-user-defined-types/85716/2 "2022-08-13T20:30:47Z")

</div>

> [@Lewis\_Hein](#):
>
> I want to be able to extend `Base.isequal` and have things like `unique` and `==` call it.

`isequal` is the one that falls back on `==`, so you want to extend `==`.

> The default implementation of isequal calls ==, so a type that does not involve floating-point values generally only needs to  
> define ==.

from `isequal`’s docstring. Also from there is:

> isequal(x,y) must imply that hash(x) == hash(y)

and `unique` depends on this behaviour too. So defining `Base.==` and `Base.hash` should get you the behaviour you want:

```julia

julia> import Base: ==, hash

julia> ==(t1::TestStruct, t2::TestStruct) = length(t1.x) == length(t2.x)
== (generic function with 170 methods)

julia> TestStruct("blah") == TestStruct("bluh")
true

julia> hash(t::TestStruct) = unsigned(length(t.x))
hash (generic function with 69 methods)

julia> unique([TestStruct("blah"), TestStruct("bluh")])
1-element Vector{TestStruct}:
 TestStruct("blah")

```