In this module, we will learn how we can write functions that return other functions. Because this seems a little weird at first, we will also discuss situations in which this is a useful design pattern, and see how this approach can be used together with Julia’s powerful dispatch system.

*Julia* has a very interesting function called `isequal`

. Let’s see how it
works (don’t forget to look at `?isequal`

for more):

```
is_two = isequal(2.0)
```

```
(::Base.Fix2{typeof(isequal), Float64}) (generic function with 1 method)
```

When we call this function, we get back *another function*. We can apply this
new function to a few arguments:

```
is_two(π)
```

```
false
```

```
is_two(2)
```

```
true
```

This is actually rather helpful! In this module, we will illustrate how
returning functions works, and see a simple example from population dynamics.
But first, why should we want to return functions? A common example is when we
want to run multiple parameterizations of a model. We *could* write the model
function in such a way that we would give it all of the parameters, but it
might lead to long function calls. What if we could have a function that only
depends on the variables? To do this, we can write a fist function to generate
the function we will actually call.

An illustration is probably more useful here. Let’s assume that we are interesting in the logistic growth of a population in discrete time, which is a simple process we can represent with the following model:

$$ n(t+1) = n(t)\times(1 + r (1 - n(t)/K)) $$

A way to simulate this could rely on a function that would be called with
something like `f(n, r, K)`

– but within a simulation, we do not expect $r$
or $K$ to change. So let’s define a function that would *only* be a function
of $n$:

```
foo(n::Float64)::Float64 = n * (1.0 + r * (1 - n / K))
```

```
foo (generic function with 1 method)
```

Now, this function will *not work* because we do not have defined `r`

or `K`

.
We can do that by having a function taking values of `r`

and `K`

as arguments,
and return a version of our function with these values “replaced”:

```
function discrete_logistic_growth(r::T, K::T)::Function where {T <: AbstractFloat}
return model(n::T)::T = n * (one(T) + r * (one(T) - n / K))
end
```

```
discrete_logistic_growth (generic function with 1 method)
```

Note that we are using a number of tricks from the previous modules: we ensure
that `r`

and `K`

have the same type (a floating point value), we let *Julia*
know that `discrete_logistic_growth`

returns a function, *and* we re-use the
type of the parameters to constrain the type of the variables.

What does it looks like in practice? We will use `Float32`

values to get a
sense that all of the annotations on our function were not in vain:

```
discrete_logistic_growth(1.0f0, 2.0f0)
```

```
(::Main.var"##339".var"#model#1"{Float32, Float32, Float32}) (generic function with 1 method)
```

Excellent, this is a generic function with a single method! We can double
check that it is, indeed, a `Function`

, by using the `isa`

operator (it also
works as a function!):

```
discrete_logistic_growth(1.0f0, 2.0f0) isa Function
```

```
true
```

Excellent! Let’s take a step back. That we are able to return functions should
not be very surprising, because functions are just another category of things.
There’s not really a lot of conceptual difference between returning a number
and returning a function. But some of the difficulty comes from the fact that
the parameters of `discrete_logistic_growth`

are now baked in the function we
return.

So how do we *use* this function? We can simply add a set of parentheses at
the end. For example, if our population has a growth rate of 1, a carrying
capacity of 2, and a current population size of 2.2, we should get a value
lower than 2:

```
discrete_logistic_growth(1.0f0, 2.0f0)(2.2f0)
```

```
1.98f0
```

Now, we can of course assign the first part of this expression to a variable:

```
parameterized_model = discrete_logistic_growth(1.0f0, 2.0f0)
```

```
(::Main.var"##339".var"#model#1"{Float32, Float32, Float32}) (generic function with 1 method)
```

We now have a fully usable function:

```
parameterized_model(2.2f0)
```

```
1.98f0
```

But *why*? Think of our function this way: as soon as it is created, using
`discrete_logistic_growth`

, we know the parameters (because we specified
them), and we know that they will not change.

Can we make this approach intersect nicely with *Julia*’s dispatch system? Of
course! Let’s assume we want to run the model sometimes on a single point (one
population), and sometimes on a series of populations (a vector of
abundances). We can do this by having, for example, one function taking a
number as input, and another function taking a vector of number as inputs. But
can we create both these functions when we create our model?

Yes!

```
function better_discrete_logistic_growth(r::T, K::T)::Function where {T <: AbstractFloat}
model(n::T)::T = n * (one(T) + r * (one(T) - n / K))
model(n::Vector{T})::Vector{T} =
n .* (one(eltype(T)) .+ r .* (one(eltype(T)) .- n ./ K))
return model
end
```

```
better_discrete_logistic_growth (generic function with 1 method)
```

Remember from the module on dispatch that a function is “just” a name, a big ol’ bag of methods. We can declare as many methods as we want, and the output is still going to be a function.

`model`

(with the
`model(n::Vector{T})::Vector{T}`

) signature uses the *dot*notation, and we will see what it is in the next module.

Let’s verify this!

```
parameterized_better_model = better_discrete_logistic_growth(0.2, 1.0)
```

```
(::Main.var"##339".var"#model#2"{Float64, Float64, Float64}) (generic function with 2 methods)
```

This is indeed a generic function with two methods. We can see it in action:

```
parameterized_better_model(0.5)
```

```
0.55
```

```
parameterized_better_model([0.0, 0.2, 1.0, 1.4])
```

```
4-element Vector{Float64}:
0.0
0.23200000000000004
1.0
1.288
```

This is another reason why the dispatch system is so important in Julia – we can use it to write a lot of different things that make the syntax of our code much more pleasant to read!