# Why no single argument \`filter(f::function)\`?
**URL:** https://discourse.julialang.org/t/why-no-single-argument-filter-f-function/92719
**Category:** Internals & Design
**Created:** [January 9, 2023, 4:01pm UTC](https://discourse.julialang.org/t/why-no-single-argument-filter-f-function/92719 "2023-01-09T16:01:43Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@jlbosse](https://discourse.julialang.org/u/jlbosse)
#### Post date: [January 9, 2023, 4:01pm UTC](https://discourse.julialang.org/t/why-no-single-argument-filter-f-function/92719/1 "2023-01-09T16:01:43Z")
Is there a good reason that there is no single-argument version of `filter` that returns a filter-function? I.e. why is the following (or something similar) not in Base
```julia
filter(f::Function) = Base.Fix1(filter, f)
```
?
This would be in line with the existence of `<(x) = (y -> y < x)` or `occursin(haystack) = (needle -> (occusin(needle, haystack)))` and very useful for data processing pipelines using `|>`.
I found [this thread](https://discourse.julialang.org/t/function-chaining-with-and-filter-function/17060/4) which disusses alternatives, but gives no reason why this shouldn’t exist in Base.
---
### Author:  [@sijo](https://discourse.julialang.org/u/sijo)
#### Post date: [January 9, 2023, 4:24pm UTC](https://discourse.julialang.org/t/why-no-single-argument-filter-f-function/92719/2 "2023-01-09T16:24:57Z")
It’s included in Julia 1.9 🙂
---
### Author:  [@jlbosse](https://discourse.julialang.org/u/jlbosse)
#### Post date: [January 9, 2023, 4:27pm UTC](https://discourse.julialang.org/t/why-no-single-argument-filter-f-function/92719/3 "2023-01-09T16:27:38Z")
Nice, looking forward to the release then!
---
### Author:  [@uniment](https://discourse.julialang.org/u/uniment)
#### Post date: [January 21, 2023, 5:54am UTC](https://discourse.julialang.org/t/why-no-single-argument-filter-f-function/92719/4 "2023-01-21T05:54:59Z")
The next question will of course be why `map` doesn’t have such a partially-applied form 😉
---
### Author:  [@jar1](https://discourse.julialang.org/u/jar1)
#### Post date: [January 21, 2023, 6:28am UTC](https://discourse.julialang.org/t/why-no-single-argument-filter-f-function/92719/5 "2023-01-21T06:28:59Z")
Because `map` is variadic, so it needs to work on 0 data arguments. I’m not sure this was the best option, but it’s what we have.
---
### Author:  [@uniment](https://discourse.julialang.org/u/uniment)
#### Post date: [January 21, 2023, 7:30am UTC](https://discourse.julialang.org/t/why-no-single-argument-filter-f-function/92719/6 "2023-01-21T07:30:09Z")
Oh wow, I was expecting it to throw an error 😅
```julia
julia> f() = 5
f (generic function with 1 method)
julia> map(f)
5
```
This behavior seems to violate any meaningful notion of mapping a collection[s] to another, and it’s not in the docstring. Under what circumstance is this useful?
For comparison:
```python
>>> def f():
... return 5
...
>>> list(map(f))
Traceback (most recent call last):
File "", line 1, in
TypeError: map() must have at least two arguments.
```
---
### Author:  [@jar1](https://discourse.julialang.org/u/jar1)
#### Post date: [January 21, 2023, 8:07am UTC](https://discourse.julialang.org/t/why-no-single-argument-filter-f-function/92719/7 "2023-01-21T08:07:34Z")
Good point. [Change map(f) behavior · Issue #48372 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/48372)
---
### Author:  [@uniment](https://discourse.julialang.org/u/uniment)
#### Post date: [January 21, 2023, 8:12am UTC](https://discourse.julialang.org/t/why-no-single-argument-filter-f-function/92719/8 "2023-01-21T08:12:49Z")
Ah, a fun reminder of my favorite Julia code:
```julia
for zip! in zip()
zip, zip, zip
zip!, zip!, zip!
end
```
---
### Author:  [@uniment](https://discourse.julialang.org/u/uniment)
#### Post date: [January 21, 2023, 8:26am UTC](https://discourse.julialang.org/t/why-no-single-argument-filter-f-function/92719/9 "2023-01-21T08:26:27Z")
It’s entertaining how [the issue](https://github.com/JuliaLang/julia/issues/35293) that it’s a duplicate of, rapidly devolves into a discussion of PR#24990 😅. Which would have it written, `map(f, _...)`.