# Value as a type parameter, \`X{1}\`
**URL:** https://discourse.julialang.org/t/value-as-a-type-parameter-x-1/74215
**Category:** New to Julia
**Tags:** question, parametric-types
**Created:** [January 7, 2022, 8:50pm UTC](https://discourse.julialang.org/t/value-as-a-type-parameter-x-1/74215 "2022-01-07T20:50:20Z")
**Posts on this page:** 10
**Page:** 1
### Author:  [@tfiers](https://discourse.julialang.org/u/tfiers)
#### Post date: [January 7, 2022, 8:50pm UTC](https://discourse.julialang.org/t/value-as-a-type-parameter-x-1/74215/1 "2022-01-07T20:50:21Z")
In “`AbstractArray{T,N}`”, I understand `T` (element type can be anything), but not `N`, the dimensionality. Why is `N` not `Int`?
---
### Author:  [@sylvaticus](https://discourse.julialang.org/u/sylvaticus)
#### Post date: [January 7, 2022, 9:00pm UTC](https://discourse.julialang.org/t/value-as-a-type-parameter-x-1/74215/2 "2022-01-07T21:00:40Z")
Indeed `Array` is a parsmetric type where the second parameter is an integer. `N` is just the name given to the parameter, it could be `Foo` or whatever…
---
### Author:  [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)
#### Post date: [January 7, 2022, 9:01pm UTC](https://discourse.julialang.org/t/value-as-a-type-parameter-x-1/74215/3 "2022-01-07T21:01:24Z")
`N` is a value, not a type. A `Vector` is `Array{T,1}`, a `Matrix` is `Array{T, 2}` etc.
---
### Author:  [@tfiers](https://discourse.julialang.org/u/tfiers)
#### Post date: [January 7, 2022, 9:21pm UTC](https://discourse.julialang.org/t/value-as-a-type-parameter-x-1/74215/4 "2022-01-07T21:21:48Z")
Ah, so parameters to types needn’t be types or type variables (like `where T`), they may be values as well.
---
### Author:  [@tfiers](https://discourse.julialang.org/u/tfiers)
#### Post date: [January 7, 2022, 9:26pm UTC](https://discourse.julialang.org/t/value-as-a-type-parameter-x-1/74215/5 "2022-01-07T21:26:38Z")
…though not any value:
```julia
julia> struct X{T} end
julia> X{8}
X{8}
julia> X{""}
ERROR: TypeError: in Type, in parameter, expected Type, got a value of type String
```
I suppose only `isbits` values can be used as a type parameter.
Then I guess my question is, is there any way to constrain value type parameters (`where {N isa Int}`?) – just like type type parameters can be constrained with `where {T <: Number}`.
---
### Author:  [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)
#### Post date: [January 7, 2022, 9:38pm UTC](https://discourse.julialang.org/t/value-as-a-type-parameter-x-1/74215/6 "2022-01-07T21:38:15Z")
You can enforce it in the constructor.
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [January 7, 2022, 9:40pm UTC](https://discourse.julialang.org/t/value-as-a-type-parameter-x-1/74215/7 "2022-01-07T21:40:53Z")
> [@tfiers](#):
>
> Why is `N` not `Int` ?
What would the dimensionality of an `Array{Float64, Int}` be?
---
### Author:  [@tfiers](https://discourse.julialang.org/u/tfiers)
#### Post date: [January 7, 2022, 9:44pm UTC](https://discourse.julialang.org/t/value-as-a-type-parameter-x-1/74215/8 "2022-01-07T21:44:22Z")
Yeah I was confused about what `N` was (a type _parameter_, not necessarily a type).
I was thinking of something like `Array{T,N} where {N isa Int}` – or indeed a constructor check like @kristoffer.carlsson says.
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [January 7, 2022, 10:00pm UTC](https://discourse.julialang.org/t/value-as-a-type-parameter-x-1/74215/9 "2022-01-07T22:00:04Z")
> [@tfiers](#):
>
> something like `Array{T,N} where {N isa Int}` – or indeed a constructor check like @kristoffer.carlsson says.
unfortunately, some times a check in the (inner) constructor is the only way to go do it as Julia doesn’t have dependent type.
---
### Author:  [@tfiers](https://discourse.julialang.org/u/tfiers)
#### Post date: [January 7, 2022, 10:59pm UTC](https://discourse.julialang.org/t/value-as-a-type-parameter-x-1/74215/10 "2022-01-07T22:59:17Z")
Whimsically, we can evaluate `Array{Int, 'π'}` no problem.
There are no constructors for it alas.