# \[ANN\] TypeCompletion.jl: exploit redundancy to complete UnionAll types

**URL:** <https://discourse.julialang.org/t/ann-typecompletion-jl-exploit-redundancy-to-complete-unionall-types/113987>\
**Category:** Package Announcements\
**Tags:** completion\
**Created:** [May 8, 2024, 8:34am UTC](https://discourse.julialang.org/t/ann-typecompletion-jl-exploit-redundancy-to-complete-unionall-types/113987 "2024-05-08T08:34:18Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![nsajko](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nsajko/32/221187_2.png) [@nsajko](https://discourse.julialang.org/u/nsajko)\
**Post date:** [May 8, 2024, 8:34am UTC](https://discourse.julialang.org/t/ann-typecompletion-jl-exploit-redundancy-to-complete-unionall-types/113987/1 "2024-05-08T08:34:18Z")

</div>

Repo: [Neven Sajko / TypeCompletion.jl · GitLab](https://gitlab.com/nsajko/TypeCompletion.jl)

General registry PR: [New package: TypeCompletion v1.0.0 by JuliaRegistrator · Pull Request #106387 · JuliaRegistries/General · GitHub](https://github.com/JuliaRegistries/General/pull/106387)

As documented (in the doc strings), overload `complete_overload` and then call `complete(your_type)`.

Just gonna copy the README.md below. NB: the JuliaHub links below will only be able to work _after_ registration.

## TypeCompletion.jl

A Julia package providing an interface for completing types. Useful for a type with  
redundancy in its definition.

### When would I want to implement the TypeCompletion.jl interface for my type?

The TypeCompletion.jl interface is useful for a type `T` such that every valid instance  
`v` of `T` must also be an instance of `S` and `S <: T`. In such a case it makes sense to  
define:

```julia
TypeCompletion.complete_overload(::Type{T}) = S

```

### Example 1: redundancy introduced to constrain the values of some parameters

Suppose you have a type `E1` and `E1` has a type parameter `N`. Further suppose that `N`  
should always be a nonnegative integer (perhaps it encodes a length). In such a case it  
might make sense to introduce an additional redundant type parameter for extra type  
safety: `R<:NTuple{N,Nothing}` (the `Nothing` is arbitrary). For example:

```julia
struct E1{Length,R<:NTuple{Length,Nothing}} end

```

`E1{L}` could ideally be completed to `E1{L,NTuple{L,Nothing}}`, likewise with  
`E1{<:Any,NTuple{L,Nothing}}`. To enable these completions, define:

```julia
const nonredundant_alias = E1{L,NTuple{L,Nothing}} where {L}
TypeCompletion.complete_overload(::Type{E1{L}}) where {L} = nonredundant_alias{L}
TypeCompletion.complete_overload(::Type{E1{<:Any,NTuple{L,Nothing}}}) where {L} = nonredundant_alias{L}

```

Implementing `TypeCompletion.complete_overload` helps minimize the inconvenience to  
users caused by the existence of the redundant type parameter.

### Example 2: redundancy introduced to define subtyping relative to an abstract type

Suppose you have a type `E2` such that `E2{A}` should subtype `AbstractVector{A}`. This  
necessitates having `A` as one of the type parameters. Further suppose that `E2` wraps  
some other `AbstractVector{A}` value, so we also want another type parameter,  
`B<:AbstractVector{A}`. Thus there is redundancy in the type parameters of `E2`, so  
TypeCompletion.jl might be useful. Suppose, then, this is the definition of `E2`:

```julia
struct E2{A, B<:AbstractVector{A}} <: AbstractVector{A} end

```

`E2{<:Any,Vector{Int}}` could ideally be completed to `E2{Int,Vector{Int}}`. To enable  
this, define:

```julia
function TypeCompletion.complete_overload(::Type{E2{<:Any,B}}) where {A,B<:AbstractVector{A}}
  E2{A,B}
end

```

---

<div class="post-metadata">

**Author:** ![nsajko](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nsajko/32/221187_2.png) [@nsajko](https://discourse.julialang.org/u/nsajko)\
**Post date:** [May 8, 2024, 8:41am UTC](https://discourse.julialang.org/t/ann-typecompletion-jl-exploit-redundancy-to-complete-unionall-types/113987/2 "2024-05-08T08:41:07Z")

</div>

This is used in the StaticViews.jl package (which will be submitted for registration after this one is registered). So look here for more example usage: [Neven Sajko / StaticViews.jl · GitLab](https://gitlab.com/nsajko/StaticViews.jl)