# Restrict type of value type parameter

**URL:** <https://discourse.julialang.org/t/restrict-type-of-value-type-parameter/12127>\
**Category:** General Usage\
**Tags:** question\
**Created:** [July 3, 2018, 1:44pm UTC](https://discourse.julialang.org/t/restrict-type-of-value-type-parameter/12127 "2018-07-03T13:44:47Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![e3c6](https://avatars.discourse-cdn.com/v4/letter/e/e79b87/32.png) [@e3c6](https://discourse.julialang.org/u/e3c6)\
**Post date:** [July 3, 2018, 1:44pm UTC](https://discourse.julialang.org/t/restrict-type-of-value-type-parameter/12127/1 "2018-07-03T13:44:47Z")

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I am declaring a parametric type `struct A{L} end`, where `L` should be an integer. For example, in `NTuple{N,T}`, the `N` parameter is restricted to be an integer. What is the recommended way to achieve this behavior on my custom time? I could not find the source code of `NTuple`.

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**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:** [July 3, 2018, 2:27pm UTC](https://discourse.julialang.org/t/restrict-type-of-value-type-parameter/12127/2 "2018-07-03T14:27:44Z")

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I think that `NTuple` _per se_ does not check that `N` is an integer, `Vararg` (which it is aliased to) does.

For `struct`s, the recommended solution is to check in the inner constructor, eg

```julia
struct A{L}
    function A{L}() where L
        @assert L isa Integer
        new{L}()
    end
end

A(L::Integer) = A{L}() # convenience constructor

A(1) # OK
A("a fish") # error

```

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<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:** [July 3, 2018, 2:30pm UTC](https://discourse.julialang.org/t/restrict-type-of-value-type-parameter/12127/3 "2018-07-03T14:30:36Z")

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PS: the source of `NTuple` is [in boot.jl](https://github.com/JuliaLang/julia/blob/175e6744b4bc9572a3575ff7946ec1e1d83ae553/base/boot.jl#L387). It is nothing more than

```julia
const NTuple{N,T} = Tuple{Vararg{T,N}}

```

There is no magic. Being able to write such core constructs in Julia lends a certain elegance to the language.

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<div class="post-metadata">

**Author:** ![e3c6](https://avatars.discourse-cdn.com/v4/letter/e/e79b87/32.png) [@e3c6](https://discourse.julialang.org/u/e3c6)\
**Post date:** [July 3, 2018, 2:52pm UTC](https://discourse.julialang.org/t/restrict-type-of-value-type-parameter/12127/5 "2018-07-03T14:52:35Z")

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Thanks. If `Vararg` is also in Julia, do you know where is the source?

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**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:** [July 3, 2018, 2:54pm UTC](https://discourse.julialang.org/t/restrict-type-of-value-type-parameter/12127/6 "2018-07-03T14:54:35Z")

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I think that `Vararg` is implemented in C.

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**Author:** ![aitzkora](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/aitzkora/32/14610_2.png) [@aitzkora](https://discourse.julialang.org/u/aitzkora)\
**Post date:** [March 2, 2021, 2:44pm UTC](https://discourse.julialang.org/t/restrict-type-of-value-type-parameter/12127/7 "2021-03-02T14:44:31Z")

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Hi, I want to achieve the same goal except that L must limited to three integer values 1,2, 3. Is there a way to that using `Union{Val{1},Val{2},Val{3}}`. In fact, after create the type I want implement dispatch  
on these values like in the constructor of Array{T,N}

Array{T,1}(::UndefInitializer, m::Int) where {T} =  
ccall(:jl\_alloc\_array\_1d, Array{T,1}, (Any, Int), Array{T,1}, m)  
Array{T,2}(::UndefInitializer, m::Int, n::Int) where {T} =  
ccall(:jl\_alloc\_array\_2d, Array{T,2}, (Any, Int, Int), Array{T,2}, m, n)

But Sadly I do not find the definition of Array{T,N} using @edit

Bests

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<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:** [March 10, 2021, 10:21am UTC](https://discourse.julialang.org/t/restrict-type-of-value-type-parameter/12127/8 "2021-03-10T10:21:23Z")

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It is not clear to me what you are doing (please open a separate topic with an MWE if you need further help). In any case, validating in the constructor is still the recommended method.

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>
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<div class="post-metadata">

**Author:** ![aitzkora](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/aitzkora/32/14610_2.png) [@aitzkora](https://discourse.julialang.org/u/aitzkora)\
**Post date:** [March 11, 2021, 10:06pm UTC](https://discourse.julialang.org/t/restrict-type-of-value-type-parameter/12127/9 "2021-03-11T22:06:04Z")

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Hi,  
Effectively, I enter too abruptly into the discussion. Next time, I will follow the rules for discourse 😄 . In addition, my question was not clear. I mixed up two points : restrict the parameter values and enable dispatch respective to the different values of the parameter. Anyway, I found a convienent solution for me and I use your trick for restricting values. Thanks