# Why doesn't empty call similar?

**URL:** <https://discourse.julialang.org/t/why-doesnt-empty-call-similar/93871>\
**Category:** Internals & Design\
**Tags:** arrays\
**Created:** [February 1, 2023, 1:17pm UTC](https://discourse.julialang.org/t/why-doesnt-empty-call-similar/93871 "2023-02-01T13:17:35Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![jishnub](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jishnub/32/33620_2.png) [@jishnub](https://discourse.julialang.org/u/jishnub)\
**Post date:** [February 1, 2023, 1:17pm UTC](https://discourse.julialang.org/t/why-doesnt-empty-call-similar/93871/1 "2023-02-01T13:17:35Z")

</div>

The docstring for empty states that

```julia

  empty(v::AbstractVector, [eltype])

  Create an empty vector similar to v, optionally changing the eltype.

```

The method, though, is defined as

```julia
empty(a::AbstractVector{T}, ::Type{U}=T) where {T,U} = Vector{U}()

```

This works, but I wonder why it doesn’t call `similar(a, U, 0)` instead? Wouldn’t this be more in line with the docstring? And it should fall back to a `Vector` anyway, if no specializations are available.

---

<div class="post-metadata">

**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [February 1, 2023, 4:29pm UTC](https://discourse.julialang.org/t/why-doesnt-empty-call-similar/93871/2 "2023-02-01T16:29:47Z")

</div>

> <https://github.com/JuliaLang/julia/pull/24390>
>
> This work relates to #24019, the theme of which is making the indexing interface… for \`AbstractArray\` and \`Associative\` much more similar (which I hope will make working with containers more seamless). 
> 
> For arrays, \`similar\` returns arrays with the same \`keys\`, whereas for dictionaries, \`similar\` returns empty dictionaries. If we are going to make the semantics of indexing and so-on more consistent between arrays and dicts, then to me it makes sense to have:
> 
> \* An \`empty\` function which is the functional companion of mutating \`empty!\` and predicate \`isempty\`, and replaces \`similar(::Associative)\`.
> \* A \`similar\` function that always preserves the existence of indices. \*Occasionally\* this might be a nice performance hack for creating new \`Dict\`s, but much more importantly this will be very useful for user-defined \`Associative\`s which may have immutable or expensive-to-insert keys, but freely mutable values.
> 
> I'm a bit new to the \`Associative\` code base... Please note that the signatures I've created for \`empty\` and \`similar\` make more sense in a world where \`Associatives\` iterate values, not key-value \`Pair\`s, which is another breaking (v1.0) suggestion in #24019.

I’m not sure; seems like a good idea to use `similar` here.