You don't have to define a type for Json data in Haskell if you don't want to. Defining a full type is only norm/preference.

With Aeson, You always have the option work with a map directly and pull out properties by name.

That is also especially useful when you can't predict what keys or shape the data will have

I don't want to detract from Aeson. It's a great library. If you control both ends of your channel, it can be more convenient than the equivalent encoding and decoding on Javascript, and making a JSON library that is more convenient than JS is no small feat.

But it's a very complex library. It may not show at your code interface, but that complexity is there at the documentation and type errors. It is also famous for generating bad runtime messages. There is even another library for fixing this, at the cost of even more complexity.

If you fall outside of its optimum usage scenario, it won't afford you more functionality than a low level parser, so you are much better saving that complexity and going with the parser.

Ok, so use a parser then? Not sure what this has to do with comparison to dynlang/js.

OP was complaining that it's hard to type some kinds of payloads.

Sure you could write a custom parser for it.

With fromJSON in fact this is what you write. Maybe if you've only been driving Json instances generically, you never actually written a fromJSON instance in Aeson? I only use generic deriving in the simplest cases.. otherwise just write the fromJSON instance parser

My point was that you can defer parsing up front and just return a map of you don't want deal with the shape.

But sure you could write a custom parser as well, that falls more into the typed philosophy though than what js folks are used to

Fair enough, you're probably right, I have only used it in contexts where I build both sides of the application.