# Type constructor ambiguities

**URL:** <https://discourse.julialang.org/t/type-constructor-ambiguities/7264>\
**Category:** General Usage\
**Tags:** type\
**Created:** [November 23, 2017, 10:00am UTC](https://discourse.julialang.org/t/type-constructor-ambiguities/7264 "2017-11-23T10:00:14Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [November 23, 2017, 10:00am UTC](https://discourse.julialang.org/t/type-constructor-ambiguities/7264/1 "2017-11-23T10:00:14Z")

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I am a bit mystified by the behaviour I am seeing when creating types that inherit from `StaticArrays.FieldVector`:

```julia
julia> using StaticArrays

julia> struct Dummy{T} <: FieldVector{2, T}
           x::T
           y::T
       end

julia> Dummy(v::AbstractVector) = Dummy(v...)
Dummy

julia> Dummy([1,2])
2-element Dummy{Int64}:
 1
 2

julia> Dummy(SVector(1,2))
ERROR: MethodError: Dummy(::SVector{2,Int64}) is ambiguous. Candidates:
  (::Type{Dummy})(v::AbstractArray{T,1} where T) in Main at REPL[4]:1
  (::Type{SA})(a::StaticArrays.StaticArray) where SA<:StaticArrays.StaticArray in StaticArrays at /Users/dnf/.julia/v0.6/StaticArrays/src/convert.jl:4
Possible fix, define
  Dummy(::StaticArrays.StaticArray{S,T,1} where T where S<:Tuple)

```

The stacktrace tells me (I think) that the `Dummy` constructor isn’t treated as a regular function/method, but that instead, something like (pseudocode)

```julia
call(::Type{Dummy}, ::SVector)

```

is being invoked. In that case I see why there is an ambiguity. I do, however, find the behaviour _very_ surprising. Even though it’s not strange that type constructors are a bit special, it seems odd that when I explicitly call `Dummy()`, that dispatch would not prioritize one of the `Dummy()` methods, if it has a matching signature.

Can someone explain the rationale behind this behaviour? It seems like an ambiguity trap, where I potentially may need special case code for all sorts of `AbstractArray`s that defines constructors similar to `StaticArray`. I’d really like to be able to create a constructor that would work on _any_ `AbstractArray`, but I don’t see how that is possible.

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<div class="post-metadata">

**Author:** ![mauro3](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mauro3/32/292_2.png) [@mauro3](https://discourse.julialang.org/u/mauro3)\
**Post date:** [November 23, 2017, 10:07am UTC](https://discourse.julialang.org/t/type-constructor-ambiguities/7264/2 "2017-11-23T10:07:54Z")

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Yep, this caught me by surprise too, see this issue: [https://github.com/FugroRoames/Rotations.jl/issues/49](https://github.com/FugroRoames/Rotations.jl/issues/49). Your explanation is correct, the type is just another slot in the method arguments and is dispatched on accordingly. However I agree that it is confusing and that special casing constructors could make sense. Well, now is the time to file an issue, or wait for Julia 2.0 (as such a change would be breaking).

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<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [November 23, 2017, 10:31am UTC](https://discourse.julialang.org/t/type-constructor-ambiguities/7264/3 "2017-11-23T10:31:34Z")

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I gave it a try: [https://github.com/JuliaLang/julia/issues/24723](https://github.com/JuliaLang/julia/issues/24723)