I hear they are good, make it easier to maintain code-bases. Most often I reach for python to get the job done. Does anyone have experiences with functional languages for larger projects?
In particular I am interested to learn more on how to handle databases, and writing to them and what patterns they come up with. Is a database handle you can write to not … basically mutable state, the arch-nemesis of functional languages?
Are functional languages only useful with an imperative shell?
To answer the headline: Functional languages are as useful in pretty much any context non-functional ones are, they just need to be handled a little different.
Functional languages’ goal tend to lean towards static or predictable typing and reduction of clandestine mutable state changes. State has to change in a program, it’s just about how its handled, and this is more about reducing referential update behavior.
Like in Python, you can have a list, pass that to a function that returns None, but deep inside that function appends to the list, and you may be surprised that your list is different as a result. This would (typically) not happen in an FP language, you would have to explicitly return the changed list up the stack of functions, and as a dev you could see from a glance that the list could change as a result of the function, leading to less surprises.
“IO” is of course an outlier, as it’s leaving the safety bubble the language provided. I could read the same database row twice in 60 seconds and get a different result, because someone else could make a transaction in that time. But once I have read the row and parsed it into a data-model, I can expect that my model in-code won’t change unless I tell it to.