error creating empty MVector

[stevengj]

This seems like it should work:

julia> MVector(SVector{0,Float64}())
ERROR: UndefVarError: T not defined
Stacktrace:
 [1] MArray{Tuple{0},T,1,0} where T(::Tuple{}) at /Users/stevenj/.julia/packages/StaticArrays/3ENSR/src/MVector.jl:20
 [2] Type at /Users/stevenj/.julia/packages/StaticArrays/3ENSR/src/MVector.jl:19 [inlined]
 [3] MArray{Tuple{S},T,1,S} where T where S(::SArray{Tuple{0},Float64,1,0}) at /Users/stevenj/.julia/packages/StaticArrays/3ENSR/src/convert.jl:5
 [4] top-level scope at none:0

[andyferris]

That does look like a bug.

In general, we are having trouble with the semantic of one argument constructors - does the user want to convert an array or nest it?

Finally - I’m curious, what do you do with an SVector{0} in practice?

[stevengj]

I encountered this for generic functionality in HCubature.jl: it has an n-dimensional SVector that it converts to an MVector for mutation purposes. The same code still needs to work in the case of n=0 since there is a well-defined answer (a 0 dimensional integral of the 0-argument function f() is just the value f()), even though the loops are empty in that case.

I worked around this by adding an explicit n=0 check, but it would be nicer if a length-0 MVector just worked in order to support edge cases like this.

[c42f]

The reverse SVector(MVector{0,Float64}()) also fails.