# How to get the container type of a container?

**URL:** https://discourse.julialang.org/t/how-to-get-the-container-type-of-a-container/20253
**Category:** General Usage
**Tags:** type, parametric-types
**Created:** [January 29, 2019, 10:25pm UTC](https://discourse.julialang.org/t/how-to-get-the-container-type-of-a-container/20253 "2019-01-29T22:25:05Z")
**Posts on this page:** 1
**Showing post:** 8

<div class="post-metadata">

### Author: ![MilesCranmer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/milescranmer/32/21070_2.png) [@MilesCranmer](https://discourse.julialang.org/u/MilesCranmer)
#### Post date: [June 15, 2023, 8:37am UTC](https://discourse.julialang.org/t/how-to-get-the-container-type-of-a-container/20253/8 "2023-06-15T08:37:37Z")

</div>

I’ve landed on this thread many, many times so I’ll just paste my modified solution here. The above version was not general enough for me, since I wanted to get the container of an arbitrary user-defined type, not just `AbstractArray`s.

I don’t remember where I saw this recursive solution originally (I think maybe the Julia docs, evolved from [this post](https://discourse.julialang.org/t/stripping-parameter-from-parametric-types/8293/2)?), but here is the code I’ve been using, with some modifications to make it instantaneous

```julia
function _container_type(T::Type)
    isa(T, UnionAll) && return _container_type(T.body)
    return T.name.wrapper
end

@generated function container_type(::Type{T}) where {T}
    container = _container_type(T)
    return :($container)
end

```

The reason it’s inside a `@generated` is just for caching. You can’t do `(::Type{T}) where {T}` in the `_container_type` because Julia will hit a stack overflow.

With this, you can get the container for arbitrary types, with no runtime cost:

```julia
julia> container_type(AbstractArray)
AbstractArray

julia> container_type(AbstractArray{Float64,3})
AbstractArray

julia> struct F{T1,T2,T3}
           x::T1
           y::T2
           z::T3
       end

julia> container_type(F{Float64,String,Tuple{Float64,Float32}})
F

julia> @btime container_type(F{Float64,String})
  0.875 ns (0 allocations: 0 bytes)
F

```

However, this will breaks if there is some abstract type with more type parameters than the user-defined struct.

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