# How to calculate the typeof an expression without actually running it?
**URL:** https://discourse.julialang.org/t/how-to-calculate-the-typeof-an-expression-without-actually-running-it/7211
**Category:** General Usage
**Created:** [November 21, 2017, 4:42am UTC](https://discourse.julialang.org/t/how-to-calculate-the-typeof-an-expression-without-actually-running-it/7211 "2017-11-21T04:42:24Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@Qiyamah](https://discourse.julialang.org/u/Qiyamah)
#### Post date: [November 21, 2017, 4:42am UTC](https://discourse.julialang.org/t/how-to-calculate-the-typeof-an-expression-without-actually-running-it/7211/1 "2017-11-21T04:42:24Z")
Hi,
I would like to calculate the typeof an expression without running it. For example:
```julia
function f(x::Int)
y = x+3
if x > 0
y+=1
else
y-=3
end
x+y
end
```
for example in a macro i am working with the expressions of this function and is there a way to calculate y+=1 to be type of Int?
Thanks
---
### Author:  [@yuyichao](https://discourse.julialang.org/u/yuyichao)
#### Post date: [November 21, 2017, 5:13am UTC](https://discourse.julialang.org/t/how-to-calculate-the-typeof-an-expression-without-actually-running-it/7211/2 "2017-11-21T05:13:30Z")
You can get some information from inference but it’s in general impossible.
---
### Author:  [@yuyichao](https://discourse.julialang.org/u/yuyichao)
#### Post date: [November 21, 2017, 5:14am UTC](https://discourse.julialang.org/t/how-to-calculate-the-typeof-an-expression-without-actually-running-it/7211/3 "2017-11-21T05:14:40Z")
And calling inference in your macro is almost certainly wrong.
---
### Author:  [@rdeits](https://discourse.julialang.org/u/rdeits)
#### Post date: [November 21, 2017, 5:09pm UTC](https://discourse.julialang.org/t/how-to-calculate-the-typeof-an-expression-without-actually-running-it/7211/4 "2017-11-21T17:09:39Z")
If you can elaborate a bit on what you’re trying to do, we may be able to provide some more useful advice. What is your macro doing that would require knowing the type of y inside the macro?
---
### Author:  [@jonathanBieler](https://discourse.julialang.org/u/jonathanBieler)
#### Post date: [November 21, 2017, 7:34pm UTC](https://discourse.julialang.org/t/how-to-calculate-the-typeof-an-expression-without-actually-running-it/7211/5 "2017-11-21T19:34:38Z")
Like yuyichao said this is probably not a good idea, but just for fun and education, there’s a macro (`@code_typed`) that shows you the inferred types. You can find what function it’s calling by doing:
```
macroexpand(:(@code_typed f(2)))
```
Turns out it’s calling `code_typed` in Base:
```
codeinfo = Base.code_typed(f,Tuple{Int64})[1]
```
You can get the inferred types from the `CodeInfo` object, e.g.:
```julia
function f(x)
z = 3
y = 2.0x
end
codeinfo = Base.code_typed(f,Tuple{Int64})[1]
collect(zip(codeinfo.first.slotnames, codeinfo.first.slottypes))
4-element Array{Tuple{Any,Any},1}:
(Symbol("#self#"), #f)
(:x, Int64)
(:z, Int64)
(:y, Float64)
```
---
### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [November 21, 2017, 8:42pm UTC](https://discourse.julialang.org/t/how-to-calculate-the-typeof-an-expression-without-actually-running-it/7211/6 "2017-11-21T20:42:46Z")
`Core.Inference.return_type`. See it in action here:
[https://github.com/ChrisRackauckas/VectorizedRoutines.jl/blob/master/src/julia.jl#L44](https://github.com/ChrisRackauckas/VectorizedRoutines.jl/blob/master/src/julia.jl#L44)