# Mutating with \`foldl\` undefined behaviour?
**URL:** https://discourse.julialang.org/t/mutating-with-foldl-undefined-behaviour/1659
**Category:** General Usage
**Tags:** question
**Created:** [January 24, 2017, 8:05am UTC](https://discourse.julialang.org/t/mutating-with-foldl-undefined-behaviour/1659 "2017-01-24T08:05:05Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@fengyang.wang](https://discourse.julialang.org/u/fengyang.wang)
#### Post date: [January 24, 2017, 8:05am UTC](https://discourse.julialang.org/t/mutating-with-foldl-undefined-behaviour/1659/1 "2017-01-24T08:05:05Z")
```julia
help?> foldl
search: foldl mapfoldl foldr mapfoldr
foldl(op, v0, itr)
Like reduce, but with guaranteed left associativity. v0 will be used exactly
once.
```
Is it undefined behaviour for `op` to mutate one (or both) of its arguments in the method above?
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [January 24, 2017, 11:04am UTC](https://discourse.julialang.org/t/mutating-with-foldl-undefined-behaviour/1659/2 "2017-01-24T11:04:22Z")
This is an interesting question — it is not documented explicitly, but given that the associativity is imposed, I would argue that
```julia
foldl(op, v0, itr)
```
should have a fixed evaluation order, and thus be equivalent in results and side effects to
```julia
let x = v0
for elt in itr
x = op(x, elt)
end
x
end
```
and deviations are bugs.
---
### Author:  [@jameson](https://discourse.julialang.org/u/jameson)
#### Post date: [January 25, 2017, 1:56am UTC](https://discourse.julialang.org/t/mutating-with-foldl-undefined-behaviour/1659/3 "2017-01-25T01:56:21Z")
I assume you referring to the `itr` argument? Yes, it’s undefined what happens to an iterator when you change its underlying container.
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [January 25, 2017, 7:10am UTC](https://discourse.julialang.org/t/mutating-with-foldl-undefined-behaviour/1659/4 "2017-01-25T07:10:24Z")
Then I misunderstood the question, I was thinking about something like
```julia
julia> x = [[i] for i in 1:5]
5-element Array{Array{Int64,1},1}:
[1]
[2]
[3]
[4]
[5]
julia> mutating_plus(acc, vec) = (acc=acc+vec[1]; vec[1] += 1; acc)
julia> foldl(mutating_plus, 0, x)
15
julia> x
5-element Array{Array{Int64,1},1}:
[2]
[3]
[4]
[5]
[6]
```
ie `foldl` having a well-defined traversal order, which is the same as iteration.
---
### Author:  [@fengyang.wang](https://discourse.julialang.org/u/fengyang.wang)
#### Post date: [January 26, 2017, 12:10am UTC](https://discourse.julialang.org/t/mutating-with-foldl-undefined-behaviour/1659/5 "2017-01-26T00:10:38Z")
This is what I meant. I encountered a strange issue recently through a definition of
```julia
append!(x, y) = foldl(push!, x, y)
```
for a custom data structure; the issue went away after I replaced the `foldl` with an explicit loop. I asked this question to help diagnose the problem.