Tony Morris

Ah, we agree then maybe. An implementation isn’t necessarily exposed, but you’re asking it to (a) use combinators (b) expose them (c) allow defining new primitive combinators instead of hiding, say, that a parser is a function of a certain type. That’s what pure embedding means!

Right. My take: "The library defines and exposes a small, simple, orthogonal core, that it uses to define the library functions."

This again refers to implementation. Could we spot a good/bad API without looking at its implementation?

John's criteria isn't about just library API, it's about the library as a whole.

Then could we tell a good library without looking at its implementation?

Definitely no, I know tons of great libraries without any documentation... Just look at what Kmett is producing.

Don't limit yourself to judging just by documentation. You can make a judgment based on the API and observable behavior.