# Match construction of \`Array\` without adding a type parameter

**URL:** <https://discourse.julialang.org/t/match-construction-of-array-without-adding-a-type-parameter/7204>\
**Category:** General Usage\
**Tags:** type, parametric-types\
**Created:** [November 20, 2017, 10:30pm UTC](https://discourse.julialang.org/t/match-construction-of-array-without-adding-a-type-parameter/7204 "2017-11-20T22:30:56Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![goretkin](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/goretkin/32/167_2.png) [@goretkin](https://discourse.julialang.org/u/goretkin)\
**Post date:** [November 20, 2017, 10:30pm UTC](https://discourse.julialang.org/t/match-construction-of-array-without-adding-a-type-parameter/7204/1 "2017-11-20T22:30:56Z")

</div>

`Array(Float64, 3)` in 0.6 should be `Array{Float64}(3)`.

If I wanted to make a type that, say, wraps an Array and also stores its length, I can:

```julia
mutable struct Wrap{A <: Array{T, 1} where {T}}
  a::A
  l::Int64
end

```

If I want to use the old syntax (`Wrap(Float64, n)`) to construct this wrapped array, then I can:

```julia
function Wrap(T, n)
  Wrap(Array{T}(n), n)
end

```

But I don’t see any way to use the following syntax `Wrap{Float64}(n)` to construct the type above.

If I do want that syntax to be available, (in order to be consistent with `Array`) I can by including an additional type parameter, and with an inner constructor as follows

```julia
mutable struct Wrap{T, A <: Array{T, 1}}
  a::A
  l::Int64
  function Wrap{T}(n) where {T}
    new{T, Array{T, 1}}(Array{T}(n), n)
  end
end

```

are there any other options? The first approach seems less complicated, except that it doesn’t look like how you construct `Array`.

(And if I want to dispatch on the `eltype` of the `a` field, I would need the repeated `T` in any case (as of today), right?)

---

<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [November 21, 2017, 7:34am UTC](https://discourse.julialang.org/t/match-construction-of-array-without-adding-a-type-parameter/7204/2 "2017-11-21T07:34:19Z")

</div>

> [@goretkin](#):
>
> The first approach seems less complicated, except that it doesn’t look like how you construct Array.

I am not sure I understand why that matters, unless you want `Wrap` to be syntactically interchangeable with `Array`; then it is a good choice.

> [@goretkin](#):
>
> if I want to dispatch on the eltype of the a field, I would need the repeated `T` in any case

Yes, that seems to be the easiest way to do it.

BTW, I am not sure if `length` was just for the MWE, but storing may not be necessary as it is a really cheap operation.

---

<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [November 21, 2017, 3:30pm UTC](https://discourse.julialang.org/t/match-construction-of-array-without-adding-a-type-parameter/7204/3 "2017-11-21T15:30:15Z")

</div>

You can sort of get what you want by just dropping the `<: Array` in your type parameter:

```julia
julia> struct Wrap{A}
           a::A
           l::Int64
           Wrap{T}(n::Integer) where {T} = new{Vector{T}}(Vector{T}(n), n)
       end

julia> Wrap{Int}(5)
Wrap{Array{Int64,1}}([0, 0, 0, 0, 0], 5)

julia> f(x::W) where {T <: Integer, W <: Wrap{Vector{T}}} = 5
f (generic function with 1 method)

julia> f(Wrap{Int}(5))
5

julia> f(Wrap{Float64}(5))
ERROR: MethodError: no method matching f(::Wrap{Array{Float64,1}})
Closest candidates are:
  f(::W<:Wrap{Array{T<:Integer,1}}) where {T<:Integer, W<:Wrap{Array{T<:Integer,1}}} at REPL[3]:100   

```

but I’m not sure I’d recommend this. I think your version with the explicit `T` parameter is more like what SubArray, OffsetArray, etc. do.