Issue #22132 has been updated by zverok (Victor Shepelev). TBH, in all my years with Ruby, it felt for me as its deep characteristic that "you never need `for`, think in enumerables." I believe that introducing "`for` that is useful for some complex cases" will dilute this image. Ruby is almost the only mainstream language that doesn't use `for` for most of the cases, and it is the first thing to be taught to newcomers: "you'll hardly need `for`, try to think in Enumerable methods." So, when met with a riddle like this, I prefer to start considering "what our Enumerables are missing, which would make it more convenient without introducing a new style." For the first case in the ticket, the answer seems to be `#product`: ```ruby def foo(n, v) (0...n).product(0...n).filter { _1 + _2 == v }.map { [_1, _2] } end p foo(10, 10) #=> [[1, 9], [2, 8], [3, 7], [4, 6], [5, 5], [6, 4], [7, 3], [8, 2], [9, 1]] ``` With the second example (inner iteration depends on outer) `product` isn't relevant, so I'd probably kept it in the "produce the sequence, then filter it" form: ```ruby (1..).lazy.flat_map { |z| (1..z).lazy.flat_map { |x| (x..z).lazy.map { |y| [x, y, z] } } } .filter { |x, y, z| x**2 + y**2 == z**2 }.take(3).force ``` I agree it is a bit cumbersome, but I believe that 3+ nested iterations is a rather rare/edge case; and still, we can have here a Ruby-ish "produce sequence, then filter sequence" structure, which is combinable to everything else (for example, filter condition can come here as a parameter/variable. PS: As a side note, the `product` solution wouldn't work in a form I wrote it, because for some reason we don't have `Enumerable#product` or `Enumerator::Lazy#product`, and only `Enumerator.product`. Various forms were proposed several times (#19324, #17312, #14399), but it hasn't moved forward yet. ---------------------------------------- Feature #22132: Scala-like for comprehensions https://bugs.ruby-lang.org/issues/22132#change-117782 * Author: shugo (Shugo Maeda) * Status: Open ---------------------------------------- ## Abstract How about adding an expression form of `for` that desugars into nested `flat_map`/`map` and `filter` calls. Here's an example, which computes all pairs of numbers between `0` and `n-1` whose sum is equal to a given value `v`: ```ruby def foo(n, v) for i in 0...n, j in 0...n when i + j == v then [i, j] end end p foo(10, 10) #=> [[1, 9], [2, 8], [3, 7]...] ``` The above code is desugared as follows: ```ruby def foo(n, v) (0...n).flat_map { |i| (0...n).filter { |j| i + j == v }.map { |j| [i, j] } } end p foo(10, 10) ``` ## Background and Motivation Some other languages have syntactic sugar that flattens nested code. For example, Scala has for comprehensions: ```scala def foo(n: Int, v: Int) = for i <- 0 until n j <- 0 until n if i + j == v yield (i, j) ``` Haskell has do notation: ```haskell foo :: Int -> Int -> [(Int, Int)] foo n v = do i <- [0 .. n-1] j <- [0 .. n-1] guard (i + j == v) pure (i, j) ``` Blocks are often nested deeply in Ruby, so such syntactic sugar is useful. ## Use cases For comprehensions can be used to flatten nested blocks. For example, ```ruby (1..).lazy.flat_map { |z| (1..z).lazy.flat_map { |x| (x..z).lazy.filter { |y| x**2 + y**2 == z**2 }.map { |y| [x, y, z] } } }.take(3).force ``` can be flattened as follows: ```ruby for z in (1..).lazy, x in (1..z).lazy, y in (x..z).lazy when x**2 + y**2 == z**2 then [x, y, z] end.take(3).force ``` For comprehensions can be used not only for Enumerable objects, but also for other objects with lawful `flat_map` and `map` (i.e., any [monad](https://wiki.haskell.org/index.php?title=Monad) whose `map` is the functor map derived from `flat_map`). For example, `flat_map` and `map` must cohere so that nested calls can be flattened: ```ruby m.flat_map(&f).map(&g) == m.flat_map { |x| f.(x).map(&g) } ``` ## Why `then` and `when`? Scala's `yield` conflicts with the existing `yield` keyword in Ruby, so I chose `then`. While a bare `for ... then` (no guard) reads a little unnaturally in English, Ruby already gives `then` a value-producing meaning (e.g., `Kernel#then`), so I consider it acceptable. The guard keyword is not `if` but `when`, because `if` after the source would be ambiguous with the modifier `if`. ## Limitations The right operand of `in` is `arg_value`, not `expr_value`, to avoid conflicts, so unparenthesized method calls (command calls) must be parenthesized. ## Backward compatibility * `for x in xs do ... end` (and the newline form) is unchanged: it still iterates via `each` and returns the collection. * All of the new forms (`for ... then`, `for ... ,`, `for ... when`) were `SyntaxError` before, so no existing program changes meaning. ## Implementation PoC: https://github.com/ruby/ruby/pull/17500 It's currently implemented only in `parse.y`, not in Prism yet, so requires `--parser=parse.y`. ## Open questions * Variable scope: the loop variables currently leak, same as `for ... do`. Should a comprehension instead scope them like a block? -- https://bugs.ruby-lang.org/