# f(x){T, N} where N may not exist

**URL:** <https://discourse.julialang.org/t/f-x-t-n-where-n-may-not-exist/13204>\
**Category:** General Usage\
**Created:** [August 10, 2018, 9:52pm UTC](https://discourse.julialang.org/t/f-x-t-n-where-n-may-not-exist/13204 "2018-08-10T21:52:02Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![JeffreySarnoff](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jeffreysarnoff/32/1980_2.png) [@JeffreySarnoff](https://discourse.julialang.org/u/JeffreySarnoff)\
**Post date:** [August 10, 2018, 9:52pm UTC](https://discourse.julialang.org/t/f-x-t-n-where-n-may-not-exist/13204/1 "2018-08-10T21:52:02Z")

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best to show the example.  
I see how to refactor it and work with two functions, multidispatchly. My use case is a preference for write-once algorithmics … and experimented seeking a flexible, easy-peasy union of Vector, SVector, NTuple where all constituents of these containers share type.

Is there a nice approach to this?

```julia
const VecOrNTuple{T,N} = Union{Vector{T}, NTuple{N,T}}

function fn(x::VecOrNTuple{T,N}) where {T,N}
    if isdefined(localcontext, :N) # ymmv 
       T, N
    else
       N
    end
end
```

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**Author:** ![ExpandingMan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/expandingman/32/866_2.png) [@ExpandingMan](https://discourse.julialang.org/u/ExpandingMan)\
**Post date:** [August 10, 2018, 11:44pm UTC](https://discourse.julialang.org/t/f-x-t-n-where-n-may-not-exist/13204/2 "2018-08-10T23:44:35Z")

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First of all, if I were you I would seriously consider abandoning the idea of using `NTuple` and simply using `AbstractVector` instead. An `SVector` is basically an `NTuple` anyway, so, if you want your `NTuple` to behave like a `Vector`, why not just use `SVector`? Of course, I don’t know the context of what you’re doing, so perhaps I’m missing something.

That said, if you really feel you need to do this, multiple dispatch is still your friend, you shouldn’t feel like you need to circumvent it somehow. You can always define a helper function thus:

```julia
g(x::Vector{T}) where {T} = T
g(x::NTuple{N,T}) where {N,T} = (T, N)

function fn(x::VectorOrNTuple{T,N}) where {T,N}
    τ = g(x) # this is just an ordinary tuple
    # here you can write arbitrarily complicated code using τ
end

```

And you can of course write as many functions as you want that use `g`. Note that there may be performance limitations if you start accessing the components of `\tau` with `getindex` since the compiler may not know the type of the resulting objects. I would be interested to know of constant propagation solves those, so it might be something fun to test on 0.7 or 1.0.

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<div class="post-metadata">

**Author:** ![JeffreySarnoff](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jeffreysarnoff/32/1980_2.png) [@JeffreySarnoff](https://discourse.julialang.org/u/JeffreySarnoff)\
**Post date:** [August 11, 2018, 12:05am UTC](https://discourse.julialang.org/t/f-x-t-n-where-n-may-not-exist/13204/3 "2018-08-11T00:05:13Z")

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good points – thank you for the writing and confirming something for others.