Promotion of four quantities fails

[mlhetland]

For example:

julia> promote(1cm, 1.0kg, 1s, 1N)
ERROR: MethodError: Cannot `convert` an object of type Tuple{Unitful.Quantity{…}} …

Promoting three or fewer quantities works just fine (as does promoting four or more other Julia values in general, of course).

[ajkeller34]

Interesting, this is automatically fixed in julia 0.7 and related to changes to dispatch. See JuliaLang/julia#21026 and JuliaLang/julia#23017. I'll keep this open until julia 0.7 is released but I'm not really sure what I can do about it until then. If you're in a jam, you could try defining:

Base.convert(::Type{Tuple{Vararg{Quantity{T}}}}, x::Tuple) where {T} = Base.cnvt_all(Quantity{T}, x...)

which should fix your problems, but may count as type piracy and is perhaps frowned upon.

[mlhetland]

Thanks! If it works in 0.7, that's good enough for me. For now, I just hacked a manual solution.

[ajkeller34]

Since Unitful's master branch only supports Julia 0.7, the only thing left to do is tag a release soon. I'm going to mark this as closed.

[singularitti]

Is this really fixed? I still have the same problem on Unitful.jl v0.17.0 on Julia 1.2.0.

julia> promote(1u"cm", 1.0u"kg", 1u"s", 1u"N")
ERROR: MethodError: Cannot `convert` an object of type Tuple{Quantity{Int64,𝐓,Unitful.FreeUnits{(s,),𝐓,nothing}},Quantity{Int64,𝐋*𝐌*𝐓^-2,Unitful.FreeUnits{(N,),𝐋*𝐌*𝐓^-2,nothing}}} to an object of type Tuple{Vararg{Quantity{Float64,D,U} where U where D,N} where N}

I have several other tests:

using Unitful

a = 1u"m"
b = 2.0u"eV"
c = 3.0u"J"
d = 1
e = 1.0
f = 2.0u"m"

julia> promote(a, b, c, f)
(1.0 m, 2.0 eV, 3.0 J, 2.0 m)

julia> promote(a, 1, 2.0)
(1.0 m, 1.0, 2.0)

julia> promote(1, c, b)
(1.0, 3.0 J, 2.0 eV)

julia> promote(1, c, b, d)
ERROR: MethodError: Cannot `convert` an object of type Tuple{Quantity{Float64,𝐋^2*𝐌*𝐓^-2,Unitful.FreeUnits{(eV,),𝐋^2*𝐌*𝐓^-2,nothing}},Int64} to an object of type Tuple{Vararg{Quantity{Float64,D,U} where U where D,N} where N}

julia> promote(1, c, b, a)
ERROR: MethodError: Cannot `convert` an object of type Tuple{Quantity{Float64,𝐋^2*𝐌*𝐓^-2,Unitful.FreeUnits{(eV,),𝐋^2*𝐌*𝐓^-2,nothing}},Quantity{Int64,𝐋,Unitful.FreeUnits{(m,),𝐋,nothing}}} to an object of type Tuple{Vararg{Quantity{Float64,D,U} where U where D,N} where N}

julia> promote(1, c, b, f)
(1.0, 3.0 J, 2.0 eV, 2.0 m)

julia> promote(1, c, b, e)
ERROR: MethodError: Cannot `convert` an object of type Tuple{Quantity{Float64,𝐋^2*𝐌*𝐓^-2,Unitful.FreeUnits{(eV,),𝐋^2*𝐌*𝐓^-2,nothing}},Float64} to an object of type Tuple{Vararg{Quantity{Float64,D,U} where U where D,N} where N}

I found the problem was due to this line:

function _promote(x, y, zs...)
    @_inline_meta
    R = promote_typeof(x, y, zs...)
    return (convert(R, x), convert(R, y), convert(Tuple{Vararg{R}}, zs)...)
end

The last line uses a convert on zs. And those errors:

julia> convert(Tuple{Vararg{Quantity{Float64}}}, (1, 2))
ERROR: MethodError: Cannot `convert` an object of type Tuple{Int64,Int64} to an object of type Tuple{Vararg{Quantity{Float64,D,U} where U where D,N} where N}

julia> convert(Tuple{Vararg{Quantity{Float64}}}, (1.0, 2.0))
ERROR: MethodError: Cannot `convert` an object of type Tuple{Float64,Float64} to an object of type Tuple{Vararg{Quantity{Float64,D,U} where U where D,N} where N}

julia> convert(Tuple{Vararg{Quantity{Float64}}}, (1u"m", 2.0u"kg"))
ERROR: MethodError: Cannot `convert` an object of type Tuple{Quantity{Int64,𝐋,Unitful.FreeUnits{(m,),𝐋,nothing}},Quantity{Float64,𝐌,Unitful.FreeUnits{(kg,),𝐌,nothing}}} to an object of type Tuple{Vararg{Quantity{Float64,D,U} where U where D,N} where N}

cause the above problems.

Is it a bug or we should not promote values like the above examples?