# Question on manual subsection: Extracting the type parameter from a super-type

**URL:** <https://discourse.julialang.org/t/question-on-manual-subsection-extracting-the-type-parameter-from-a-super-type/97137>\
**Category:** New to Julia\
**Tags:** question, parametric-types, parametric-methods\
**Created:** [April 5, 2023, 9:37pm UTC](https://discourse.julialang.org/t/question-on-manual-subsection-extracting-the-type-parameter-from-a-super-type/97137 "2023-04-05T21:37:26Z")\
**Posts on this page:** 1\
**Showing post:** 4

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**Author:** ![Benny](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Benny](https://discourse.julialang.org/u/Benny)\
**Post date:** [April 6, 2023, 2:14am UTC](https://discourse.julialang.org/t/question-on-manual-subsection-extracting-the-type-parameter-from-a-super-type/97137/4 "2023-04-06T02:14:33Z")

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I think the manual has a bad example. Before I explain, I think the discussion is getting a little confused; the manual is showing hypothetical source code. If you run it in your own module or REPL, those scopes are accessing your own personal `AbstractArray` and `eltype` definitions, not the ones in `Base`. Avoid the types, and rename functions if you want to try them out, e.g. `eltype` to `eltype_correct`.

Back to the point, the manual is trying to make a distinction between `Type{<:AbstractArray{T}}` and `Type{AbstractArray{T}}` in the argument annotations. The former specifies arguments being any subtype of `AbstractArray`, like `Array{Int, N} where N`, but the latter only specifies `AbstractArray` itself. `T` being in the method’s `where` clause means the inputs must provide a specific type for `T`.

The `supertype`-using `eltype_wrong` method does work on any subtype of `AbstractArray`, but it does not specify the parameter `T` we want; instead it tries to recursively `supertype` its way to `AbstractArray` so the other `eltype_wrong` methods can take it. It is indeed correct that if `supertype` doesn’t work, `eltype_wrong` doesn’t work.

However, the correct code as is wouldn’t work either because `Union{AbstractArray{Int}, AbstractArray{Float64}}` does not have a specific type for `T`, despite being a subtype of `AbstractArray`. The example has an easy fix, make the types in the `Union` share a type for `T`:

```julia
eltype_correct(Array{Int,3}) # Int
eltype_correct(Union{Array{Int,1}, Array{Int,3}}) # Int
eltype_wrong(Array{Int,3}) # Int
eltype_wrong(Union{Array{Int,1}, Array{Int,3}}) # errors

```

Incidentally, this is where it is important to specify parameters belonging to the argument versus the method. A version of the correct code for the number of dimensions would look like

```julia
my_ndims(::Type{<:AbstractArray{T,N} where T}) where {N} = N
my_ndims(Union{Array{Int,1}, Array{Bool,1}}) # 1

```

`Base.ndims` now behaves like this, but it used to error on unspecific `T` like this `Union` or `Array{T,1} where T` because its method had both parameters `{T,N}` (issue [#40682](https://github.com/JuliaLang/julia/pull/40682)).

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