Keywords are useful if:
- A function has many parameters, and remembering the order of these is hard. Using a keyword list is nice in this case, since it shows at a function call location what information is used for which purpose inside the function.
- A function has many optional parameters.
- A function takes some arguments from the list of options, and passes the rest on. (Note that while this is nice in some cases, it is often a bad idea as it does not ‘fail fast’; you won’t notice right away if you made a typo in the option name, for instance)
An important drawback of keyword lists is that when you ‘just’ use them, your function might be passed without some argument that it needs, so depending on where your function is used, you might want to add some descriptive error messages if people forget to pass it.
As you indeed already mentioned, not being able to pattern-match on them, as the keywords might be passed in any order is another drawback. Therefore, while they are nice in an outward-facing API, internally you’ll probably end up calling a high-arity function anyway.
So, my rule of thumb: Use them when it makes the public-facing code more clear, and use them when there are many optional arguments.
