> From: "Brian Goetz" <brian.go...@oracle.com> > To: "Remi Forax" <fo...@univ-mlv.fr> > Cc: "amber-spec-experts" <amber-spec-experts@openjdk.java.net> > Sent: Thursday, May 19, 2022 3:03:55 PM > Subject: Re: Collections patterns
>> We may want to extract sub-parts of the array / collections by example, and i >> would prefer to have the same semantics and a similar syntax. > This is pretty vague, so I'll have to guess about what you might mean. > Maybe you mean: "I want to match a list if it contains the a subsequence that > matches this sequence of patterns", something like: > [ ... p1, p2, ... ] > There is surely room to have APIs that query lists like this, but I think this > is way out of scope for a pattern matching feature the language. Pattern > matching is about _destructuring_. (Destructuring can be conditional.) An > array > is a linear sequence of elments; it can be destructured by a linear sequence > of > patterns. > Maybe you mean: "I want to decompose a list into the head element and a tail > list". > In Haskell, we iterate a list by recursion: > len :: [a] -> Int > len [] = 0 > len x:xs = 1 + len xs > But again, this is *mere destructuring*, because the cons operator (:) is the > linguistic primitive for aggregation, and [ ... ] lists are just sugar over > cons. So matching to `x:xs` is again destructuring. We could try to apply this > to Java, but it gets very clunky (inefficient, no tail recursion, yada yada) > because our lists are *built differently*. Further further, arrays are another > step removed from lists even. > Or maybe you mean something else; if so, please share! The current proposal is more about matching and extracting the first arguments than matching/extracting the last arguments or the rest are also useful IMO. By example, if i want to parse command line arguments composed of options and a filename, i may want to write something like case [String... options, String filename] -> ... case [String...] -> help() Rémi
