Definition of `one`

[gdalle]

I'm a bit surprised by the following behavior, is there a rationale for returning an integer instead of a unitful quantity?

julia> using Unitful

julia> x = [1u"m"]
1-element Vector{Quantity{Int64, 𝐋, Unitful.FreeUnits{(m,), 𝐋, nothing}}}:
 1 m

julia> x[1] = one(eltype(x))
ERROR: DimensionError: m and 1 are not dimensionally compatible.
Stacktrace:
 [1] convert(::Type{Quantity{Int64, 𝐋, Unitful.FreeUnits{(m,), 𝐋, nothing}}}, x::Int64)
   @ Unitful ~/.julia/packages/Unitful/GMTL8/src/conversion.jl:133
 [2] setindex!(A::Vector{Quantity{Int64, 𝐋, Unitful.FreeUnits{(m,), 𝐋, nothing}}}, x::Int64, i::Int64)
   @ Base ./array.jl:987
 [3] top-level scope
   @ ~/Documents/GitHub/Julia/_Scratchpad/mwe.jl:36

julia> x[1] == one(eltype(x))
false

julia> one(eltype(x))
1

[giordano]

That's precisely the semantic of one:

Return a multiplicative identity for x: a value such that one(x)*x == x*one(x) == x. Alternatively one(T) can take a type T, in which case one returns a multiplicative identity for any x of type T.

If possible, one(x) returns a value of the same type as x, and one(T) returns a value of type T. However, this may not be the case for types representing dimensionful quantities (e.g. time in days), since the multiplicative identity must be dimensionless. In that case, one(x) should return an identity value of the same precision (and shape, for matrices) as x.

If you want a quantity that is of the same type as x, or of type T, even if x is dimensionful, use oneunit instead.

[gdalle]

Thanks, I wasn't aware of oneunit!