The library generates code from haskell-src-exts AST. The procedure is as follows:

1. pretty-print AST generated by haskell-src-exts
2. parse the generated code to get SrcSpanInfo for each node
3. combine AST annotated with comments and AST annotated with SrcSpanInfo
4. insert (non-empty) comments into each node, updating SrcSpanInfo of all nodes
5. profit!

As you can see, the algorithm is nor remarkably fast, because it prints and parses code and modifies SrcSpanInfo of all nodes in an AST on each comment insertion. On the good side, it is quite flexible and compatible with many versions of haskell-src-exts. Performance is also acceptable if you don't need to invoke it every millisecond.