# How to reference the type of a function in a struct?
**URL:** https://discourse.julialang.org/t/how-to-reference-the-type-of-a-function-in-a-struct/16919
**Category:** New to Julia
**Tags:** question
**Created:** [October 29, 2018, 12:59pm UTC](https://discourse.julialang.org/t/how-to-reference-the-type-of-a-function-in-a-struct/16919 "2018-10-29T12:59:17Z")
**Posts on this page:** 12
**Page:** 1
### Author:  [@leiteiro](https://discourse.julialang.org/u/leiteiro)
#### Post date: [October 29, 2018, 12:59pm UTC](https://discourse.julialang.org/t/how-to-reference-the-type-of-a-function-in-a-struct/16919/1 "2018-10-29T12:59:17Z")
This is the type I want to use:
```
struct S
F::Function
n::Int
end
```
However I want to allow only functions F which are of exactly this type:
```
F(n::Int, k::Int)::BigInt
```
How can I express this in the struct S?
---
### Author:  [@mauro3](https://discourse.julialang.org/u/mauro3)
#### Post date: [October 29, 2018, 1:10pm UTC](https://discourse.julialang.org/t/how-to-reference-the-type-of-a-function-in-a-struct/16919/2 "2018-10-29T13:10:05Z")
You cannot. Julia does not encode the arguments and return-type of functions (or more like “methods”) in their type.
Side note, if you care about performance, then you should parameterize on the function-type:
```julia
struct S{T<:Function}
F::T
n::Int
end
```
---
### Author:  [@dpsanders](https://discourse.julialang.org/u/dpsanders)
#### Post date: [October 29, 2018, 1:26pm UTC](https://discourse.julialang.org/t/how-to-reference-the-type-of-a-function-in-a-struct/16919/3 "2018-10-29T13:26:39Z")
There is FunctionWrappers.jl.
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [October 29, 2018, 1:32pm UTC](https://discourse.julialang.org/t/how-to-reference-the-type-of-a-function-in-a-struct/16919/4 "2018-10-29T13:32:17Z")
You cannot express it in the struct directly, but you can enforce something like this in the constructor, by checking the output of `Base.return_types(F, (Int, Int))`.
---
### Author:  [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)
#### Post date: [October 29, 2018, 1:58pm UTC](https://discourse.julialang.org/t/how-to-reference-the-type-of-a-function-in-a-struct/16919/5 "2018-10-29T13:58:07Z")
> [@DNF](#):
>
> You cannot express it in the struct directly, but you can enforce something like this in the constructor, by checking the output of `Base.return_types(F, (Int, Int))` .
This is not really a good idea. You are tying the logic of your code to inference (which is a heuristic) and the result will be very brittle.
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [October 29, 2018, 2:00pm UTC](https://discourse.julialang.org/t/how-to-reference-the-type-of-a-function-in-a-struct/16919/6 "2018-10-29T14:00:55Z")
Is it the implementation that makes it brittle, or is the goal of restricting the input/output types of `F` inherently problematic?
---
### Author:  [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)
#### Post date: [October 29, 2018, 2:15pm UTC](https://discourse.julialang.org/t/how-to-reference-the-type-of-a-function-in-a-struct/16919/7 "2018-10-29T14:15:18Z")
Something like (updated thanks @Tamas_Papp)
```julia
struct S
F::Function
n::Int
S(F::Function, n::Int) = new( (n::Int, k::Int) -> F(n, k)::BigInt, n)
end
```
might be ok?
But any attempt to try to error when the struct `S` is created (as opposed to when `S.F` is used) seems difficult.
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [October 29, 2018, 2:36pm UTC](https://discourse.julialang.org/t/how-to-reference-the-type-of-a-function-in-a-struct/16919/8 "2018-10-29T14:36:32Z")
> [@kristoffer.carlsson](#):
>
> might be ok?
Since the closure `isa Function`, I guess you need some extra step not to get an infinite loop.
---
### Author:  [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)
#### Post date: [October 29, 2018, 2:38pm UTC](https://discourse.julialang.org/t/how-to-reference-the-type-of-a-function-in-a-struct/16919/9 "2018-10-29T14:38:53Z")
Thanks, updated.
---
### Author:  [@Bruno\_Amorim](https://discourse.julialang.org/u/Bruno_Amorim)
#### Post date: [October 29, 2018, 5:55pm UTC](https://discourse.julialang.org/t/how-to-reference-the-type-of-a-function-in-a-struct/16919/10 "2018-10-29T17:55:10Z")
Would in this case still be advantageous to parametrize the sctruc with the function-type as proposed by @mauro3 ?
---
### Author:  [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)
#### Post date: [October 29, 2018, 6:02pm UTC](https://discourse.julialang.org/t/how-to-reference-the-type-of-a-function-in-a-struct/16919/11 "2018-10-29T18:02:53Z")
Likely, yes.
---
### Author:  [@Bruno\_Amorim](https://discourse.julialang.org/u/Bruno_Amorim)
#### Post date: [October 29, 2018, 6:24pm UTC](https://discourse.julialang.org/t/how-to-reference-the-type-of-a-function-in-a-struct/16919/12 "2018-10-29T18:24:08Z")
Thank you!