# Overload call to do multiplication on numbers?
**URL:** https://discourse.julialang.org/t/overload-call-to-do-multiplication-on-numbers/9279
**Category:** General Usage
**Created:** [February 23, 2018, 1:16pm UTC](https://discourse.julialang.org/t/overload-call-to-do-multiplication-on-numbers/9279 "2018-02-23T13:16:24Z")
**Posts on this page:** 10
**Page:** 1
### Author:  [@antoine-levitt](https://discourse.julialang.org/u/antoine-levitt)
#### Post date: [February 23, 2018, 1:16pm UTC](https://discourse.julialang.org/t/overload-call-to-do-multiplication-on-numbers/9279/1 "2018-02-23T13:16:24Z")
I really like the 2x syntax, and get tired of writing stars.
edit: actual example…
```julia
function (x::Float64)(y)
x*y
end
a = 2.0
a(2+2)
```
That looks like a hilariously bad idea, but I can’t find any actual problems with this. Anything I missed?
---
### Author:  [@fredrikekre](https://discourse.julialang.org/u/fredrikekre)
#### Post date: [February 23, 2018, 1:23pm UTC](https://discourse.julialang.org/t/overload-call-to-do-multiplication-on-numbers/9279/2 "2018-02-23T13:23:13Z")
This already works without that definition though:
```julia
julia> 2.0pi
6.283185307179586
julia> 2.0(2+2)
8.0
julia> 2.0sin(pi)
2.4492935982947064e-16
```
---
### Author:  [@antoine-levitt](https://discourse.julialang.org/u/antoine-levitt)
#### Post date: [February 23, 2018, 1:31pm UTC](https://discourse.julialang.org/t/overload-call-to-do-multiplication-on-numbers/9279/3 "2018-02-23T13:31:30Z")
Sorry about that, I changed my example but didn’t test 🙂 What didn’t work without the definition was:
```julia
a = 2.0
a(2+2)
```
---
### Author:  [@fredrikekre](https://discourse.julialang.org/u/fredrikekre)
#### Post date: [February 23, 2018, 1:38pm UTC](https://discourse.julialang.org/t/overload-call-to-do-multiplication-on-numbers/9279/4 "2018-02-23T13:38:31Z")
```julia
julia> pi(2+2)
ERROR: MethodError: objects of type Irrational{:π} are not callable
julia> a = 2
2
julia> a(2+2)
ERROR: MethodError: objects of type Int64 are not callable
```
But I get what you mean.
> [@antoine-levitt](#):
>
> but I can’t find any actual problems with this.
It looks like function call syntax, so for the examples above I would expect to call a function `pi` and `a` respectively.
---
### Author:  [@antoine-levitt](https://discourse.julialang.org/u/antoine-levitt)
#### Post date: [February 23, 2018, 1:43pm UTC](https://discourse.julialang.org/t/overload-call-to-do-multiplication-on-numbers/9279/5 "2018-02-23T13:43:52Z")
Sorry again, I edited my second post (I should really wait to be on a computer before replying…)
My motivating example was writing `sin(2pi(x+y))` by accident and wondering if that could work.
---
### Author:  [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)
#### Post date: [February 23, 2018, 5:18pm UTC](https://discourse.julialang.org/t/overload-call-to-do-multiplication-on-numbers/9279/6 "2018-02-23T17:18:49Z")
It’s definitely doable, but it seems a bit dangerous. I wouldn’t want to have `sin = 666` somewhere and then write `sin(x + y)` later, thinking that I’m calling the `sin` function but actually just be multiplying `x + y` by the number of the beast.
---
### Author:  [@Mason](https://discourse.julialang.org/u/Mason)
#### Post date: [February 23, 2018, 7:25pm UTC](https://discourse.julialang.org/t/overload-call-to-do-multiplication-on-numbers/9279/7 "2018-02-23T19:25:28Z")
I really like that in Mathematica `a b` Lowers to `a*b`. This would probably be horribly invasive and dangerous to implement in Julia but man would I love to be able to write `sin(2pi (x+y))` and have the space between `2pi` and `(x+y)` be understood as multiplication.
---
### Author:  [@antoine-levitt](https://discourse.julialang.org/u/antoine-levitt)
#### Post date: [February 26, 2018, 8:30am UTC](https://discourse.julialang.org/t/overload-call-to-do-multiplication-on-numbers/9279/8 "2018-02-26T08:30:18Z")
That’s fair enough, this feature, while very cool, is also fairly dangerous. For those interested:
```julia
function build_subtypes(T)
concretes = [t for t in subtypes(T) if isempty(subtypes(t))]
abstracts = [t for t in subtypes(T) if !isempty(subtypes(t))]
types = []
append!(types, concretes)
for t in abstracts
append!(types, build_subtypes(t))
end
types
end
for T in build_subtypes(Number)
@eval begin
function (x::$T)(y)
x*y
end
end
end
```
---
### Author:  [@Per](https://discourse.julialang.org/u/Per)
#### Post date: [February 26, 2018, 8:43am UTC](https://discourse.julialang.org/t/overload-call-to-do-multiplication-on-numbers/9279/9 "2018-02-26T08:43:30Z")
Space is already used as a separator in matrices, so `sin.([2pi (x+y)])` would be ambiguous…
---
### Author:  [@Mason](https://discourse.julialang.org/u/Mason)
#### Post date: [February 26, 2018, 2:42pm UTC](https://discourse.julialang.org/t/overload-call-to-do-multiplication-on-numbers/9279/10 "2018-02-26T14:42:44Z")
Right, I had a feeling there was something that would straight up clash with spaces as multiplication. How could I forget row vectors and matrices!