# How to parameterize a type on dimensionality of the wrapped array?

**URL:** <https://discourse.julialang.org/t/how-to-parameterize-a-type-on-dimensionality-of-the-wrapped-array/68008>\
**Category:** General Usage\
**Tags:** parametric-types\
**Created:** [September 10, 2021, 8:55pm UTC](https://discourse.julialang.org/t/how-to-parameterize-a-type-on-dimensionality-of-the-wrapped-array/68008 "2021-09-10T20:55:30Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![tlnagy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tlnagy/32/5815_2.png) [@tlnagy](https://discourse.julialang.org/u/tlnagy)\
**Post date:** [September 10, 2021, 8:55pm UTC](https://discourse.julialang.org/t/how-to-parameterize-a-type-on-dimensionality-of-the-wrapped-array/68008/1 "2021-09-10T20:55:30Z")

</div>

Struggling a bit with parametric types for `TiffImages.jl`. I want to have two different types of `DenseTaggedImage` for 2D and 3D images since the `ifd` variable is a vector for 3D images and 0-dimensional for 2D images.

```julia
julia> abstract type AbstractTIFF{T, N} <: AbstractArray{T, N} end

julia> abstract type AbstractDenseTIFF{T, N} <: AbstractTIFF{T, N} end

julia> struct IFD{O <: Unsigned} end

julia> struct DenseTaggedImage{T, O <: Unsigned, AA <: AbstractArray{T, 3}} <: AbstractDenseTIFF{T, 3}
           data::AA
           ifd::Vector{IFD{O}}
       end

julia> struct DenseTaggedImage{T, O <: Unsigned, AA <: AbstractArray{T, 2}} <: AbstractDenseTIFF{T, 2}
           data::AA
           ifd::IFD{O}
       end
ERROR: invalid redefinition of constant DenseTaggedImage
Stacktrace:
 [1] top-level scope
   @ REPL[5]:1

```

I thought adding the dimension to the AbstractArray parameter and to the subtyping info would be sufficient to create a new type, but I’m still getting a redefinition error. **Where did I go wrong here?**

I used to have a single type for both 2D and 3D arrays: [https://github.com/tlnagy/TiffImages.jl/blob/70e14527c97cc019f71abdb4ab0d98416f9187b2/src/types/dense.jl#L3-L17](https://github.com/tlnagy/TiffImages.jl/blob/70e14527c97cc019f71abdb4ab0d98416f9187b2/src/types/dense.jl#L3-L17)

---

<div class="post-metadata">

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [September 11, 2021, 12:16am UTC](https://discourse.julialang.org/t/how-to-parameterize-a-type-on-dimensionality-of-the-wrapped-array/68008/2 "2021-09-11T00:16:02Z")

</div>

One possibility is to parametrize the complete type of `ifd` and put the dimension as the first parameter for dispatch. Maybe that is simpler. Something like:

> [@tlnagy](#):
>
> ```julia
> julia> struct DenseTaggedImage{N,AA,TIFD}
> data::AA
> ifd::TIFD
> end
> 
> ```