Ah. You know what? I forgot that the Java implementation of these concepts isn't stupid like it is in some other languages (except what the heck is mapToInt? Some optimized version that makes a primitive array, I guess? Yucky- I wish the compiler could just figure that out).

So, I concede that Java's addition of the stream API is a legitimately good example of adding an aspect of functional programming to an otherwise very non-FP language.

But, let me go off on my tangent, anyway. ;)

It's not that you need to convince me that functional programming is great. It's just that I find that consistent and coherent designs tend to work well and that kitchen-sink or be-everything-to-everybody approaches tend to be good at nothing and mediocre-to-bad at everything.

MOST languages that have tacked on the low-hanging fruit of FP (map, filter, etc combinators on collections) have done it in a really sub-optimal way.

JavaScript, for example. JavaScript has eager, mutable, non-persistent, arrays as the default collection data structure. When they added map, reduce, filter, etc to Array, they added them in the most naive possible way, which means that doing something like your example above (map-then-sum), would create an entire extra array with the same number of elements as the original, and would end up looping both arrays once. So we have ~2N memory usage and 2N iterations where we really should just have an extra 8 bytes to hold the sum and iterate over the array once (N iterations).

Same thing with other languages like Swift and Kotlin.

Kotlin maybe should have an asterisk because it has Sequence, which will mostly work like Java's streams. However, there are two issues: it still offers them on eager iterables, instead of forcing us to use a sequence/stream to access them, and with suspend functions you have to be careful with Sequences. In you Java example, we're theoretically allocating a new Stream object with every combinator call, BUT we "know" that the compiler is smart enough to avoid those allocations and the result code will be about as fast as writing a for-loop. With Kotlin's suspend functions, we can very easily thwart the compiler's ability to do that. If you use a Sequence chain inside a suspend function and call another suspend function as part of that chain, then that's a yield point and the compiler can no longer optimize away the allocation of the intermediate Sequence object(s).

So, my point is that designing a language with some initial philosophy and then trying to borrow from, frankly, incompatible other philosophies usually leads to sub-optimal implementations and/or APIs. Again, though, Java's streams are a good counter example to my claim.

> I (and the pretty much every language designer in the post-Java era) disagree with you about checked exceptions, but that's a whole different thread...

Indeed it is! :) I'm willing to be the black sheep, and die on that hill, though (too many metaphors?). And, honestly, I don't think it's as unanimous as some people claim. I see returning monadic error values as isomorphic to checked exceptions, and several languages have gone that route since Java: Scala, Swift, and Rust, to name a few. Kotlin's lead dude, Roman, simultaneously claims that checked exceptions were a terrible mistake, but then also advocates for using sealed classes for return values when failure is expected or in the domain, which sounds a lot like what checked exceptions are supposed to be used for. TypeScript can't have monadic error handling because of its design philosophy of being a thin layer over JavaScript, but many in that community have embraced using union types for return values instead of throwing Errors.

Cheers!