# Question on composite types
**URL:** https://discourse.julialang.org/t/question-on-composite-types/43661
**Category:** New to Julia
**Tags:** question
**Created:** [July 25, 2020, 11:26am UTC](https://discourse.julialang.org/t/question-on-composite-types/43661 "2020-07-25T11:26:46Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@janrpeters](https://discourse.julialang.org/u/janrpeters)
#### Post date: [July 25, 2020, 11:26am UTC](https://discourse.julialang.org/t/question-on-composite-types/43661/1 "2020-07-25T11:26:46Z")
Is there a method that returns
```julia
Float64
```
if given the composite type
```julia
Point{Float64}
```
?
is
---
### Author:  [@tomerarnon](https://discourse.julialang.org/u/tomerarnon)
#### Post date: [July 25, 2020, 11:55am UTC](https://discourse.julialang.org/t/question-on-composite-types/43661/2 "2020-07-25T11:55:02Z")
You can get the parameters of a concrete type with:
```julia
julia> Vector{Int}.parameters
svec(Int64, 1)
```
---
### Author:  [@tisztamo](https://discourse.julialang.org/u/tisztamo)
#### Post date: [July 25, 2020, 12:02pm UTC](https://discourse.julialang.org/t/question-on-composite-types/43661/3 "2020-07-25T12:02:41Z")
If you want to start from an instance, then a simple function can be used to extract it:
```julia
julia> struct Point{T}
a::T
end
julia> p=Point{Int}(42)
Point{Int64}(42)
julia> pointparam(::Point{T}) where T = T
pointparam (generic function with 1 method)
julia> pointparam(p)
Int64
```
---
### Author:  [@tbeason](https://discourse.julialang.org/u/tbeason)
#### Post date: [July 25, 2020, 3:20pm UTC](https://discourse.julialang.org/t/question-on-composite-types/43661/4 "2020-07-25T15:20:06Z")
This is what `eltype` is usually used for. It doesn’t come for free for custom structs, so if it is your struct you need to do
```julia
Base.eltype(::Point{T}) where {T} = T
```
which is the same function as the previous post but you are sticking to the common usage here.
If you are using someone else’s package (or base Julia types), `eltype` should already be defined.
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [July 25, 2020, 5:45pm UTC](https://discourse.julialang.org/t/question-on-composite-types/43661/5 "2020-07-25T17:45:59Z")
> [@tbeason](#):
>
> This is what `eltype` is usually used for.
For iterables, yes, but that may not be the case here.
> [@tbeason](#):
>
> If you are using someone else’s package (or base Julia types), `eltype` should already be defined.
`eltype` has a catch-all method that falls back to `Any`, so in a sense it is always defined. However, it is only required to be correctly implemented if the relevant parts of the [iterator interface](https://docs.julialang.org/en/v1/manual/interfaces/) apply.