# Specify the type of function when it is a parameter of another function
**URL:** https://discourse.julialang.org/t/specify-the-type-of-function-when-it-is-a-parameter-of-another-function/85724
**Category:** General Usage
**Tags:** question, functors, function-parameters
**Created:** [August 14, 2022, 10:34am UTC](https://discourse.julialang.org/t/specify-the-type-of-function-when-it-is-a-parameter-of-another-function/85724 "2022-08-14T10:34:59Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@rocco\_sprmnt21](https://discourse.julialang.org/u/rocco_sprmnt21)
#### Post date: [August 14, 2022, 10:34am UTC](https://discourse.julialang.org/t/specify-the-type-of-function-when-it-is-a-parameter-of-another-function/85724/1 "2022-08-14T10:34:59Z")
Perhaps the question doesn’t matter much in practice or even makes little sense in theory. In which case, please explain the reasons in a simple way.
Playing with the splitarray function, I wondered if and how it was possible to impose constraints on the dlm function.
In particular, the fact that it must have elements of type T in input and a Boolean value in output.
```julia
function splitarray(a:: Vector{T},dlm::Function) where T
spl=findall(dlm,a)
getindex.([a], (:).([1; spl.+ 1], [spl.- 1; lastindex(a)]))
end
```
---
### Author:  [@digital\_carver](https://discourse.julialang.org/u/digital_carver)
#### Post date: [August 14, 2022, 1:28pm UTC](https://discourse.julialang.org/t/specify-the-type-of-function-when-it-is-a-parameter-of-another-function/85724/2 "2022-08-14T13:28:48Z")
I don’t think so. A function’s type is `typeof(functionname)`, and that is a direct subtype of `Function`.
```julia
julia> typeof(sin) |> supertypes
(typeof(sin), Function, Any)
```
So there’s no place in the type hierarchy that specifies its input or output types.
Within your function, you can do a `hasmethod` check if needed, but generally you’ll get a `MethodError` if the function can’t accept the right types anyway, so that’s usually unnecessary.
---
### Author:  [@hendri54](https://discourse.julialang.org/u/hendri54)
#### Post date: [August 14, 2022, 5:42pm UTC](https://discourse.julialang.org/t/specify-the-type-of-function-when-it-is-a-parameter-of-another-function/85724/3 "2022-08-14T17:42:10Z")
What are you trying to accomplish? Do you wish to dispatch on properties of the function? Or just check whether the function has the right signature before calling `findall`?
---
### Author:  [@rocco\_sprmnt21](https://discourse.julialang.org/u/rocco_sprmnt21)
#### Post date: [August 14, 2022, 6:29pm UTC](https://discourse.julialang.org/t/specify-the-type-of-function-when-it-is-a-parameter-of-another-function/85724/4 "2022-08-14T18:29:14Z")
> [@hendri54](#):
>
> Or just check whether the function has the right signature before calling `findall`?
Just this^ check.
As I said it is just an exercise to experiment with various language features.
I was wondering if it was possible to filter the input parameters without making explicit checks within the function.
But, if I am not mistaken, in fact `dlm` is the same predicate that is used as a parameter by `findall` and, as far as I can understand / see, it does not make these restrictions.
PS
however, I would be curious to know if, even with some unorthodox artifice, it is possible to impose these restrictions.
For example by defining a new structure that contains only functions with those characteristics.
---
### Author:  [@hendri54](https://discourse.julialang.org/u/hendri54)
#### Post date: [August 14, 2022, 6:56pm UTC](https://discourse.julialang.org/t/specify-the-type-of-function-when-it-is-a-parameter-of-another-function/85724/5 "2022-08-14T18:56:12Z")
You could use a functor.
```julia
# Define abstract type for the class of functions deemed acceptable.
abstract type MyFunction{T} <: Function end
# Functor
struct F1{T} <: MyFunction{T}
x :: T
end
function (f :: F1{T})(z :: T) where T
f.x + z;
end
fct1 = F1(1.0);
@assert fct1 isa MyFunction{Float64};
@assert fct1(2.0) ≈ 3.0
function bar(f :: MyFunction{T}, z :: AbstractVector{T}) where T
findall(y -> y < f.x, z)
end
@show bar(fct1, [0.5, 0.7, 1.2, 2.3])
```