# Promote\_type of nested tuple elements
**URL:** https://discourse.julialang.org/t/promote-type-of-nested-tuple-elements/2761
**Category:** General Usage
**Created:** [March 19, 2017, 3:02pm UTC](https://discourse.julialang.org/t/promote-type-of-nested-tuple-elements/2761 "2017-03-19T15:02:47Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@gasagna](https://discourse.julialang.org/u/gasagna)
#### Post date: [March 19, 2017, 3:02pm UTC](https://discourse.julialang.org/t/promote-type-of-nested-tuple-elements/2761/1 "2017-03-19T15:02:47Z")
I have nested tuples like
```julia
tup = Tuple{Tuple{Int64,Int8},Tuple{Int128,Int32}}
```
and I need to find the largest eltype of the elements, i.e. `Int128` in the above case. I feel I need to use `promote_type` in a recursive fashion, but my attempts have failed so far.
Thanks!
---
### Author:  [@dpsanders](https://discourse.julialang.org/u/dpsanders)
#### Post date: [March 19, 2017, 3:18pm UTC](https://discourse.julialang.org/t/promote-type-of-nested-tuple-elements/2761/2 "2017-03-19T15:18:18Z")
What are your attempts so far?
---
### Author:  [@gasagna](https://discourse.julialang.org/u/gasagna)
#### Post date: [March 19, 2017, 3:31pm UTC](https://discourse.julialang.org/t/promote-type-of-nested-tuple-elements/2761/3 "2017-03-19T15:31:24Z")
I think I got it. Here it is
```julia
using Base.Test
promote_tuple_type(a::Number) = typeof(a)
promote_tuple_type(a::Tuple) = promote_type(map(promote_tuple_type, a)...)
# basic tests
tup0 = ((1, 1), (1, 2), (2, 3), (2, 3, 4), ((2, 3, (3, 4, (5, 5, (1,))))))
tup1 = ((Int8(1), Int32(1)), (Int64(1), Float64(1)))
tup2 = ((Int8(1), Int32(1)), (Int64(1), Int128(1)))
tup3 = (((Int8(1), Int32(1)), (Int64(1), Int128(1))), (Float32(1), Float16(1)))
@test promote_tuple_type(tup0) == Int64
@test promote_tuple_type(tup1) == Float64
@test promote_tuple_type(tup2) == Int128
@test promote_tuple_type(tup3) == Float32
```
This works well, but maybe there is better way of doing it.
---
### Author:  [@gasagna](https://discourse.julialang.org/u/gasagna)
#### Post date: [March 19, 2017, 3:45pm UTC](https://discourse.julialang.org/t/promote-type-of-nested-tuple-elements/2761/4 "2017-03-19T15:45:14Z")
The related problem is to construct a new tuple where all elements have the same type. I came up with two options
```julia
convert_tuple{T}(::Type{T}, a::Number) = T(a)
convert_tuple{T}(::Type{T}, a::Tuple) = (map(el->convert_tuple(T, el), a)...)
convert_tuple{T}(::Type{T}, a::Tuple) = (convert_tuple.(T, a)...)
```
The second looks nicer but incurs in more allocations than the first.
These works OK for me, but again, there might be a better way of doing it.