Type alias Transformer<Input, Output>

Transformers handle syntax trees and files.

They are functions that are called each time a syntax tree and file are passed through the run phase. When an error occurs in them (either because it’s thrown, returned, rejected, or passed to next), the process stops.

The run phase is handled by trough, see its documentation for the exact semantics of these functions.

👉 Note: you should likely ignore next: don’t accept it. it supports callback-style async work. But promises are likely easier to reason about.