what to do about one and zero ?

[jlapeyre]

Currently, we have this:

julia> one(Diagonal(fill(1.0, 2)))    # 1
2×2 Array{Float64,2}:
 1.0  0.0
 0.0  1.0

julia> zero(Diagonal(fill(1.0, 2)))  # 2
2×2 Diagonal{Float64,Array{Float64,1}}:
 0.0   ⋅ 
  ⋅   0.0

julia> one(Diagonal(Fill(1.0, 2)))  # 3
2×2 Array{Float64,2}:
 1.0  0.0
 0.0  1.0

julia> zero(Diagonal(Fill(1.0, 2)))  # 4
2×2 Diagonal{Float64,Array{Float64,1}}:
 0.0   ⋅ 
  ⋅   0.0

one and zero behave differently, but they probably should behave analogously.
The documentation for one says

If possible, one(x) returns a value of the same type as x,

And for zero

additive identity element for the type of x

This says to me that, of the above examples, only number 2 is correct.

[dlfivefifty]

I think just return the most structured version, so one returns Eye and zero returns Zeros.

[dlfivefifty]

This is partially fixed:

julia> one(Diagonal(Fill(1.0, 2)))  # 3
2×2 Diagonal{Float64,Array{Float64,1}}:
 1.0   ⋅ 
  ⋅   1.0

julia> zero(Diagonal(Fill(1.0, 2))) 
2×2 Diagonal{Float64,Array{Float64,1}}:
 0.0   ⋅ 
  ⋅   0.0

but still, would be better to return Eye and Zeros

[jishnub]

This is largely resolved now:

julia> one(Diagonal(Fill(1.0, 2)))
2×2 Diagonal{Float64, Fill{Float64, 1, Tuple{Base.OneTo{Int64}}}}:
 1.0   ⋅ 
  ⋅   1.0

julia> zero(Diagonal(Fill(1.0, 2)))
2×2 Diagonal{Float64, Fill{Float64, 1, Tuple{Base.OneTo{Int64}}}}:
 0.0   ⋅ 
  ⋅   0.0

[dlfivefifty]

Is it good that they are Fill, not Ones / Zeros ?

[jishnub]

Maybe Ones or Zeros would be better, so this may be improved. This was raised here as well.