I touched on the idea of API ergonomics in Values for Collaborative Codebases, but I think the topic is due a bit more exploration. Typically you think about an API as being "safe" or "functionally complete," or "easy to use," but "ergonomic" is a bit far afield from the standard way that people think and talk about APIs (in my experience.)
I think part of the confusion is that "API" gets used in a couple of different contexts, but let's say that an API here are the collection of nouns (types, structures,) and verbs (methods, functions) used to interact with a concept (hardware, library, service). APIs can be conceptually really large (e.g. all of a database, a public service), or quite small and expose only a few simple methods (e.g. a data serialization library, or some kind of hashing process.) I think some of the confusion is that people also use the term API to refer to the ways that services access data (e.g. REST, etc.) and while I have no objection to this formulation, service API design and class or library API design feel like related but different problems.
Ergonomics, then is really about making choices in the design of an API, so that:
- functionality is discoverable during programming. If you're writing in a language with good code completion tools, then make sure methods and functions are well located and named in a way to take advantage of completion. Chainable APIs are awesome for this.
- use clear naming for functions and arguments that describe your intent and their use.
- types should imply semantic intent. If your programming language has a sense of mutability (e.g. passing references verses concrete types in Go, or const (for all its failings) in C++), then make sure you use these markers to both enforce correct behavior and communicate intent.
- do whatever you can to encourage appropriate use and discourage inappropriate use, by taking advantage of encapsulation features (interfaces, non-exported/private functions, etc.), and passing data into and out of the API with strongly/explicitly-typed objects (e.g. return POD classes, or enumerated values or similar rather than numeric or string types.)
- reduce the complexity of the surface area by exporting the smallest reasonable API, and also avoiding ambiguous situations, as with functions that take more than one argument of a given type, which leads to cases where users can easily (and legally) do the wrong thing.
- increase safety of the API by removing or reducing and being explicit about the API's use of global state. Avoid providing APIs that are not thread safe. Avoid throwing exceptions (or equivalents) in your API that you expect users to handle. If users pass nil pointers into an API, its OK to throw an exception (or let the runtime do it,) but there shouldn't be exceptions that originate in your code that need to be handled outside of it.
Ergonomic interfaces feel good to use, but they also improve quality across the ecosystem of connected products.