In the past it was difficult to work with the GHC AST as any generic traversals had to carefully tiptoe around an assortment of panic and undefined expressions. These have now been removed, by uses of PostRn, PostTc, etc, allowing standard traversal libraries to be used. See module hsSyn/PlaceHolder.
The relevant commit is 7d3f2dfc7a45d741224c521e0f2a616a89f9506f
Introduce an annotation structure to the ParsedSource to record the location of un-captured keywords (Alan Zimmerman).
At the moment the location of let / in / if / then / else / do etc is not captured in the AST. This makes it difficult to parse some source, transform the AST, and then output it again preserving the original layout.