dtl-js/docs/helpers/chain.md

Data Transformation Language - JSON templates and data transformation

## `chain()` Function

### Description
The `chain()` function in DTL is used to apply a sequence of data transformations in a specific order. It is particularly useful within transform libraries, where it can be employed to execute multiple named transformations in succession.

### Usage
```dtl
chain( $data, $transform_chain )
```
- `$data`: The initial input data to be transformed.
- `$transform_chain`: An array that represents the sequence of transformations to apply to the data.

### Details
- Each element in the `$transform_chain` represents a step in the transformation process.
- The `$data` is passed through each transform or expression in the `$transform_chain` one after the other.
- This method enables the composition of complex data transformations in a modular and readable manner.

### Example
Consider a scenario where you have a string `$greeting` with the value 'hello'. You want to capitalize it and then reverse it. You can define a `transform_chain` like so:

```dtl
{
    "out": "(: chain($greeting, 'transform_chain') :)",
    "transform_chain": [
        "(: capitalize($.) :)",
        "(: reverse($.) :)"
    ]
}
```
The `chain()` will apply the transformations in order, resulting in 'olleH' from the initial `$greeting` value 'hello'.

### Considerations
- The sequence in which transforms are placed in the `$transform_chain` is critical, as each transform's output will be the input for the next.
- It is important to ensure that each transform is compatible with the output of the previous one to prevent errors or unintended results.

### Performance Implications
- When using `chain()`, the performance should be considered if individual transforms are resource-intensive or if the data set is large.

### Note
- `chain()` enhances the readability and maintainability of complex data transformation sequences.
- While it abstracts away complexity, developers should still be mindful of the logical flow and compatibility of transformations within the chain.
```