Collections patterns

Brian Goetz brian.goetz at
Thu May 19 13:03:55 UTC 2022

> 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!
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