I really like it. I particularly like that you've resisted the temptation to include SQL itself or the jq queries in the DSL.
The validation piece makes it feel a bit a bit like the Rails mindset for people who work better in FP.
I'd make a could of suggestions for the docs:
Maybe a bit more discussion of how we'd test our webpipe code.
I see why you've called them 'middlewares' but, maybe the term 'macros' or 'pipeline functions' might avoid confusion with express/connect middlewares
The validation piece makes it feel a bit a bit like the Rails mindset for people who work better in FP.
I'd make a could of suggestions for the docs: Maybe a bit more discussion of how we'd test our webpipe code. I see why you've called them 'middlewares' but, maybe the term 'macros' or 'pipeline functions' might avoid confusion with express/connect middlewares