We all ran into the use case where we have an array on which we want to :

- filter elements
- apply a transformation on each of the filtered elements

Consider the following example:

**We have an array of numbers on which we want to double only the even numbers.**

The easiest way of writing the later would be:

```
const double = (n:number) => n * 2
const isEven = (n:number) => n % 2 === 0
const doubleTheEven = (numbers:number[]) => numbers.filter(isEven).map(double)
doubleTheEven([1, 2, 3, 4]) // [4, 8] β
```

That works fine but is not awesome performance wise, since **we are iterating through the whole array twice**.

A way to get around that perf problem would be to write our `doubleTheEven`

function with some imperative code like this:

```
const doubleTheEven = (numbers:number[]) => {
const result = [];
for (const number of numbers) {
if (isEven(number)) {
result.push(double(number));
}
}
return result;
};
```

That fixes the performance issue but reduces our code quality and scalability (suppose we want to chain multiple maps and filters, and in any order we like).

##
**Transducers** are the tool that will allow us to get around that compromise π

First lets explicit some definitions that we'll be using.

`map`

is a method that takes a **transformation** of type:

```
type Transformation<T, W> = (a: T) => W;
// takes a value of type T and returns of value of type W.
// our transformation "double" has type Transformation<number, number>
```

`filter`

method takes a **predicate**:

```
type Predicate<T> = (a: T) => boolean;
```

`reduce`

method takes a **reducer**:

```
type Reducer<A, C> = (acc: A, curr: C) => A;
```

A transducer takes a reducer and returns a reducer, thus has type:

```
type Transducer<A, C> = (r: Reducer<A, C>) => Reducer<A, C>;
```

We'll also need the compose function available in any FP library near you:

```
const compose = (...fns: Function[]) => (x: any) =>
fns.reduceRight((y, f) => f(y), x);
compose(n => n * 2, n => n + 1)(2) // 6
compose(n => n + 1, n => n * 2)(2) // 5
```

## Now let's get to work π€

We'll first need a reducer that takes a value and pushes it to the accumulator.

```
const pushReducer = (accumulator, current) => [...accumulator, current];
```

Naturally, since `pushReducer`

doesn't do anything to the `current`

value, when we reduce an array with `pushReducer`

, we get back the same array.

```
[1,2,3].reduce(pushReducer,[]) // [1,2,3] β
```

(spoiler alertπ¨: our transducers are going to tap into those values to transform or filter them before they get to the `pushReducer`

π).

Remember how we said transducers take a reducer and return a reducer ?

We are going to apply transducers to `pushReducer`

, and those transducers will do transformation or filtering.

### Creating transducers for transforming

Let's write a function that takes a transformation, and returns a transducer that applies that transformation to the `current`

value for the provided reducer:

```
const makeXfTransducer = (xf: Transformation): Transducer => reducer => (
acc,
curr
) => reducer(acc, xf(curr));
```

Let's apply it to `double`

:

```
const doubleTransducer = makeXfTransducer(double);
```

So, when we apply `doubleTransducer`

to `pushReducer`

, we get back a reducer that does the same thing as `pushReducer`

but with `xf`

applied to `curr`

.

Let's verify that:

```
const reducerThatDoubles = doubleTransducer(pushReducer);
[1, 2, 3, 4].reduce(reducerThatDoubles), []); // [2, 4, 6, 8] β
```

### Creating transducers for filtering

We need need a similar function that handles filters and here it is:

```
const makeFilterTransducer = (pred: Predicate): Transducer => reducer => (
acc,
curr
) => (pred(curr) ? reducer(acc, curr) : acc);
```

Let's smoke test it:

```
[1, 2, 3, 4].reduce(isEvenTransducer(pushReducer), []); // [2, 4] β
```

### Composing transducers

Now you probably saw this coming: we are going **compose** those transducers!

```
const doubleEvenReducer = compose(
isEvenTransducer,
doubleTransducer
)(pushToReducer);
[1, 2, 3, 4].reduce(doubleEvenReducer, []); // [4, 8] β
```

And there you have it π

But wait, it's sort of a pain to have too explicit the use of the `pushReducer`

everytime...

Let's write a more concise way of doing all this that we'll call `into`

.

It has the following type and implementation:

```
type Into<A, C> = (t: A, x: Transducer<A, C>, i: A) => A;
// In our case A = number[] and C = number
const into: Into = (target, transducer, input) =>
input.reduce(transducer(pushReducer), target);
```

And we see that everything works:

```
into(
[],
compose(
isEvenTransducer,
doubleTransducer
),
[1, 2, 3, 4]
); // [4, 8] β
```

## Implement transducers with Ramda

Obviously we invented nothing here and the Ramda library already provides all these utilities out of the box

```
import { into, compose, map, filter } from "ramda";
const input = [1, 2, 3, 4];
const expected = [4, 8];
const doubleTransducer = map(double);
const isEvenTransducer = filter(isEven);
into(
[],
compose(
isEvenTransducer,
doubleTransducer
),
[1, 2, 3, 4]
);
```

All code examples can be found here

Happy coding ! π»

## Discussion (1)

Do you if it's possible to type

`makeXfTransducer`

in typescript?