I like the idea but the example doesn't make much sense.
In what application would you load all users into memory from database and then filter them with TypeScript functions? And that is the problem with the otherwise sound idea "Functional core, imperative shell". The shell penetrates the core.
Maybe some filters don't match the way database is laid out, what if you have a lot of users, how do you deal with email batching and error handing?
So you have to write the functional core with the side effect context in mind, for example using query builder or DSL that matches the database conventions. Then weave it with the intricacies of your email sender logic, maybe you want iterator over the right size batches of emails to send at once, can it send multiple batches in parallel?
I am surprised by this example, for the same reason.
Generally, performance is a top cause of abstraction leaks and the emergence of less-than-beautiful code. On an infinitely powerful machine it would be easy and advisable to program using neat abstracrions, using purely "the language of" the business. Our machines are not infinitely powerful, and that is especially evident when larger data sets are involved. That's where, to achieve useful performance, you have to increasingly speak "the language of" the machine. This is inevitable, and the big part of the programmer's skill is to be able to speak both "languages", to know when to speak which one, and produce readable code regardless.
Database programming is a prime example. There's a reason, for example, why ORMs are very messy and constitute such excellent footguns: they try to gap this bridge, but inevitably fail in important ways. And having and ORM in the example would, most likely, violate the "functional core" principle from the article.
So it looks like the author accidentally presented a very good counterexample to their own idea. I like the idea though, and I would love to know how to resolve the issue.
In what application would you load all users into memory from database and then filter them with TypeScript functions? And that is the problem with the otherwise sound idea "Functional core, imperative shell". The shell penetrates the core.
Maybe some filters don't match the way database is laid out, what if you have a lot of users, how do you deal with email batching and error handing?
So you have to write the functional core with the side effect context in mind, for example using query builder or DSL that matches the database conventions. Then weave it with the intricacies of your email sender logic, maybe you want iterator over the right size batches of emails to send at once, can it send multiple batches in parallel?