### when

Tests a value, `x`, against a predicate function. If `true`, return `fn(x)`. Else, return `x`. 

Return a function expecting a single value, `x`, that returns the appropriate value based on `pred`.

```js
const when = (pred, whenTrue) => (x) => pred(x) ? whenTrue(x) : x;
```

```js
const doubleEvenNumbers = when(
  (x) => x % 2 === 0,
  (x) => x * 2
);
doubleEvenNumbers(2); // 4
doubleEvenNumbers(1); // 1
```