# Promote on ArbFloat issue?
**URL:** https://discourse.julialang.org/t/promote-on-arbfloat-issue/102839
**Category:** Numerics
**Tags:** question
**Created:** [August 15, 2023, 7:10pm UTC](https://discourse.julialang.org/t/promote-on-arbfloat-issue/102839 "2023-08-15T19:10:38Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@roi.holtzman](https://discourse.julialang.org/u/roi.holtzman)
#### Post date: [August 15, 2023, 7:10pm UTC](https://discourse.julialang.org/t/promote-on-arbfloat-issue/102839/1 "2023-08-15T19:10:38Z")
I am trying to make sure that all of my `ArbFloat`s are with the same precision.
I do not understand why promote works like that:
```julia
a = ArbFloat(2, digits=200)
typeof(a)
# ArbFloat{691}
typeof.(promote(a, ArbFloat(0.0)))
# (ArbFloat{128}, ArbFloat{128})
```
I would expect that `ArbFloat(0.0)` would be promoted to `ArbFloat{691}`. Why is it not so?
---
### Author:  [@oheil](https://discourse.julialang.org/u/oheil)
#### Post date: [August 16, 2023, 10:23am UTC](https://discourse.julialang.org/t/promote-on-arbfloat-issue/102839/2 "2023-08-16T10:23:56Z")
It is implemented to do so:
```julia
promote_rule(::Type{ArbFloat{P}},::Type{ArbFloat{Q}}) where {P,Q} = P
### Author:  [@oheil](https://discourse.julialang.org/u/oheil)
#### Post date: [August 16, 2023, 10:30am UTC](https://discourse.julialang.org/t/promote-on-arbfloat-issue/102839/3 "2023-08-16T10:30:23Z")
My guess is: results of operations on arguments with different precision should have the lowest precision of all the arguments. Everything else would pretend a precision which isn’t there.
---
### Author:  [@JeffreySarnoff](https://discourse.julialang.org/u/JeffreySarnoff)
#### Post date: [August 16, 2023, 11:58am UTC](https://discourse.julialang.org/t/promote-on-arbfloat-issue/102839/4 "2023-08-16T11:58:04Z")
makes sense to me
---
### Author:  [@roi.holtzman](https://discourse.julialang.org/u/roi.holtzman)
#### Post date: [August 16, 2023, 1:08pm UTC](https://discourse.julialang.org/t/promote-on-arbfloat-issue/102839/5 "2023-08-16T13:08:03Z")
You’re right!
That does make sense.
I guess that I would like to have the lower precision number passed with zeros at the end.
For example, if I have something like this:
```julia
a=ArbFloat(0.2, bits=100)
b=ArbFloat(0.3, bits=1000)
function f(x, y)
a, b = promote(a, b)
a * b
end
f(a, b)
```
I would like `a` to be padded with zeros.
---
### Author:  [@JeffreySarnoff](https://discourse.julialang.org/u/JeffreySarnoff)
#### Post date: [August 30, 2023, 9:27pm UTC](https://discourse.julialang.org/t/promote-on-arbfloat-issue/102839/6 "2023-08-30T21:27:25Z")
```julia
using ArbNumerics
const ArbFR = Union{ArbFloat, ArbReal}
promote_widen(a, b) = promote(a, b)
function promote_widen(a::ArbFR, b::ArbFR)
aprec, bprec = precision(a), precision(b)
if aprec == bprec
return a, b
end
if aprec < bprec
a = typeof(b)(a)
else
b = typeof(a)(b)
end
a, b
end
function times(a, b)
a, b = promote_widen(a, b)
a * b
end
```
then
```julia
a = 0.3f0
b = 0.7
x = ArbFloat(0.3, bits = 50)
y = ArbFloat(0.7, bits = 100)
ab = times(a, b);
bx = times(b, x);
xy = times(x, y);
a = 0.3f0
b = 0.7
x = ArbFloat(0.3, bits = 50)
y = ArbFloat(0.7, bits = 100)
aa = times(a, a);
ab = times(a, b);
bx = times(b, x);
xy = times(x, y);
```
we obtain
```julia
julia> precision(aa), aa
(24, 0.09f0)
julia> precision(ab), ab
(53, 0.21000000834465027)
julia> precision(bx), bx
(50, 0.21)
julia> precision(xy), xy
(100, 0.209999999999999978905762532122)
```
```julia
```