There is a great Steve Yegge post on how useful ad-hoc transformation of source code is: http://steve-yegge.blogspot.com/2007/06/rich-programmer-food...

The only time I have used this myself was an expat style transformer for terraform (HCL). We had a lot of terraform and they kept changing the language, so I would build a fixer to make code written for say 0.10 to work with 0.12 and then again for 0.14. It was very fun and let us keep updating to newer terraform versions. Pretty simple language except for distinguishing quoted blocks from non-quoted.