– advanced question –

Let’s say, I have a `Vector{Int}`

and would like to transform it into a `Vector{Float64}`

.

One super generic way to do it is to use the hidden return_type function `Core.Compiler.return_type`

. Namely with the following

```
Core.Compiler.return_type(map, Tuple{typeof(x -> 0.3*x), Vector{Int}}) # Array{Float64,1}
```

The pitfall is that I have to give a concrete function… Hence the question:

**Can I construct a FunctionType with a given Input and a given Output Type without actually providing an implementation?**

It should make the following work

```
Core.Compiler.return_type(map, Tuple{FunctionType(Int, Float64), Vector{Int}}) # Array{Float64,1}
```