# \`in\` as variable name leads to calling of variable

**URL:** <https://discourse.julialang.org/t/in-as-variable-name-leads-to-calling-of-variable/133733>\
**Category:** Internals & Design\
**Tags:** question, keywords\
**Created:** [November 7, 2025, 1:02pm UTC](https://discourse.julialang.org/t/in-as-variable-name-leads-to-calling-of-variable/133733 "2025-11-07T13:02:08Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![ncxst](https://avatars.discourse-cdn.com/v4/letter/n/e47774/32.png) [@ncxst](https://discourse.julialang.org/u/ncxst)\
**Post date:** [November 7, 2025, 1:02pm UTC](https://discourse.julialang.org/t/in-as-variable-name-leads-to-calling-of-variable/133733/1 "2025-11-07T13:02:08Z")

</div>

I stumbled across code that uses `in` as a variable name, which confused me, so I tried to see when using keywords as variables breaks. First a few examples where everything is fine:

```julia-auto
julia> f(in, out) = in + out
f (generic function with 1 method)

julia> f(1,2)
3

julia> f(in, ∈) = in + ∈
f (generic function with 1 method)

julia> f(1,2)
3

julia> f(i, in) = i ∈ in
f (generic function with 1 method)

julia> f(1,[1,2])
true

julia> f(i, ∈) = i in ∈
f (generic function with 1 method)

julia> f(1,[1,2])
true

```

As long as the variable is not used as a keyword too everything works fine. But when `in` is used both as a variable and keyword, the variable is getting called:

```julia-auto
julia> f(i, in) = i in in
f (generic function with 1 method)

julia> f(1,[1,2])
ERROR: MethodError: objects of type Vector{Int64} are not callable
Use square brackets [] for indexing an Array.
The object of type `Vector{Int64}` exists, but no method is defined for this combination of argument types when trying to treat it as a callable object.
Stacktrace:
 [1] f(i::Int64, in::Vector{Int64})
   @ Main ./REPL[9]:1
 [2] top-level scope
   @ REPL[10]:1

```

Is this the expected behaviour?

Other keywords are not allowed as variable names:

```julia-auto
julia> f(for) = for
ERROR: ParseError:
# Error @ REPL[11]:1:6
f(for) = for
# ╙ ── unexpected `)`
Stacktrace:
 [1] top-level scope
   @ REPL:1

julia> f(end) = end
ERROR: ParseError:
# Error @ REPL[12]:1:3
f(end) = end
# └ ── Expected `)` or `,`
Stacktrace:
 [1] top-level scope
   @ REPL:1

julia> f(if) = if
ERROR: ParseError:
# Error @ REPL[13]:1:5
f(if) = if
# ╙ ── unexpected `)`
Stacktrace:
 [1] top-level scope
   @ REPL:1

```

I see that users might like to use `in` as a variable for an input, but it leads to weird behavior as shown above. So my question is should `in` be allowed as a variable name?

---

<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [November 7, 2025, 1:09pm UTC](https://discourse.julialang.org/t/in-as-variable-name-leads-to-calling-of-variable/133733/2 "2025-11-07T13:09:10Z")

</div>

> [@ncxst](#):
>
> I see that users might like to use `in` as a variable for an input, but it leads to weird behavior as shown above. So my question is should `in` be allowed as a variable name?

`in` is only a keyword in the syntax of `for` loops and comprehensions. Otherwise it is an ordinary identifier that works as a binary operator, in the same way that `+` is an ordinary identifier that works as a binary operator.

For example:

```julia-auto
julia> typeof(in) # the builtin `Base.in` is an identifier for an infix function
typeof(in) (singleton type of function in, subtype of Function)

julia> f(in) = [x^2 for x in in] # "in" used as both a local variable and a keyword
f (generic function with 1 method)

julia> f(1:3)
3-element Vector{Int64}:
 1
 4
 9

julia> g(i, in) = i in 1 # "in" used as a binary operator, not a keyword
g (generic function with 1 method)

julia> g(2, +)
3

```

Of course, I wouldn’t recommend using `in` in this way, in the same way that I wouldn’t recommend redefining `+` to be `17` or `atan`.