# Constructors for parameterized arrays
**URL:** https://discourse.julialang.org/t/constructors-for-parameterized-arrays/9030
**Category:** New to Julia
**Created:** [February 13, 2018, 10:22am UTC](https://discourse.julialang.org/t/constructors-for-parameterized-arrays/9030 "2018-02-13T10:22:49Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@stephancb](https://discourse.julialang.org/u/stephancb)
#### Post date: [February 13, 2018, 10:22am UTC](https://discourse.julialang.org/t/constructors-for-parameterized-arrays/9030/1 "2018-02-13T10:22:49Z")
Trying to code the generation of an array as a constructor, for example,
`Vector{Float64}(f::Function, n::Int) = [f(2*pi*(k-1)/n) for k in 1:n]`
gives a `WARNING: static parameter Float64 does not occur ...` and, of course, for example
`Vector{Float64}(sin, 16)`
doesn’t work.
Yes, there are zillions of other ways to do this in Julia, but I’m just wondering, how comes that `Base.Vector{Float64}(n::Int)` is working (it is obviously not implemented in pure Julia?)?
BTW, after the first try in my REPL
`Vector{Float64}(3)` does not work any more. So trying to overload the Array constructor in this (faulty) manner seems to do more serious damage than being just not callable.
(This is Julia 0.62, Fedora 27)
---
### Author:  [@fredrikekre](https://discourse.julialang.org/u/fredrikekre)
#### Post date: [February 13, 2018, 10:45am UTC](https://discourse.julialang.org/t/constructors-for-parameterized-arrays/9030/2 "2018-02-13T10:45:55Z")
```julia
julia> (::Type{Vector{Float64}})(f::Function, n::Int) = [f(2*pi*(k-1)/n) for k in 1:n]
julia> Vector{Float64}(sin, 4)
4-element Array{Float64,1}:
0.0
1.0
1.22465e-16
-1.0
```
---
### Author:  [@stephancb](https://discourse.julialang.org/u/stephancb)
#### Post date: [February 13, 2018, 10:56am UTC](https://discourse.julialang.org/t/constructors-for-parameterized-arrays/9030/3 "2018-02-13T10:56:45Z")
Ah, thanks. Probably also `Base.Vector{Float64}(n::Int)` is then implemented similarly.
My “BTW” remains. Rephrased, should the damage by trying to overload constructors the wrong way be avoided, e.g. by throwing an error instead of just a warning?
---
### Author:  [@fredrikekre](https://discourse.julialang.org/u/fredrikekre)
#### Post date: [February 13, 2018, 11:04am UTC](https://discourse.julialang.org/t/constructors-for-parameterized-arrays/9030/4 "2018-02-13T11:04:34Z")
It works for me:
```julia
julia> (::Type{Vector{Float64}})(f::Function, n::Int) = [f(2*pi*(k-1)/n) for k in 1:n]
julia> Vector{Float64}(sin, 4)
4-element Array{Float64,1}:
0.0
1.0
1.22465e-16
-1.0
julia> Vector{Float64}(4)
4-element Array{Float64,1}:
6.90402e-310
6.90401e-310
6.90401e-310
0.0
```
> [@stephancb](#):
>
> Rephrased, should the damage by trying to overload constructors the wrong way be avoided, e.g. by throwing an error instead of just a warning?
The general advice is to not overload types that you don’t own. (See e.g. [https://docs.julialang.org/en/latest/manual/style-guide/#Avoid-type-piracy-1](https://docs.julialang.org/en/latest/manual/style-guide/#Avoid-type-piracy-1)). If you do, you may run into things like
[Segfault on a simple method definition · Issue #20945 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/20945)
[Ungraceful exit when redefining Base.promote\_type · Issue #22132 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/22132)
[fatal and strange: errors when overwriting Base.\<, Base.==, etc. for Int64,Float64,etc. using @eval in v0.6.0 · Issue #23605 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/23605)
[Stack Overflow (or segfault) with: (a=\>b) = (1=\>2) · Issue #21944 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/21944)
[julia REPL aborting after harmless syntax error · Issue #24078 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/24078)
[Segfault on method error · Issue #24122 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/24122)
[Base.getproperty with Dict type gets fatal error · Issue #25208 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/25208)
[(::Type{T})(x::Int) where {T\<:Integer} = println("crash") · Issue #25740 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/25740)
---
### Author:  [@stephancb](https://discourse.julialang.org/u/stephancb)
#### Post date: [February 13, 2018, 11:33am UTC](https://discourse.julialang.org/t/constructors-for-parameterized-arrays/9030/5 "2018-02-13T11:33:24Z")
Works now for me, too.
Sorry for the false alarm. I still have the non-working `Vector{Float64}(3)` in the terminal buffer, but can’t reproduce it any more. Something else way before must have damaged my REPL session.
Thanks for the advice. The planned use is having as first parameter a type that I own, i.e. calling the constructor like:
`floatdata=Vector{Float64}(h::Handle_to_nonfloat_data, ... )`
Perhaps not too bad then, but certainly a matter of taste.
---
### Author:  [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)
#### Post date: [February 13, 2018, 11:39am UTC](https://discourse.julialang.org/t/constructors-for-parameterized-arrays/9030/6 "2018-02-13T11:39:26Z")
I _think_ that after the 0.7 deprecations are removed, this will be written as:
```julia
Vector{Float64}(f::Function, n::Int) = ...
```
which is nicer.
---
### Author:  [@stephancb](https://discourse.julialang.org/u/stephancb)
#### Post date: [February 13, 2018, 11:46am UTC](https://discourse.julialang.org/t/constructors-for-parameterized-arrays/9030/7 "2018-02-13T11:46:28Z")
Yes, nicer