Issue #22132 has been updated by ko1 (Koichi Sasada). 1. It seems simpler to use `each` with accumulation like that: ```ruby def foo(n, v) for i in 0...n, j in 0...n when i + j == v then [i, j] end end #=> def foo(n, v) _a = [] i = j = nil (0...n).each do i = it (0...n).each do j = it if i + j == v then _a << [i, j] end end end end ``` 2. how about `break`? I think it should escape `for` syntax. It should like: ``` def foo(n, v) for i in 0...n, j in 0...n when i + j == v then break if cond [i, j] end end #=> def foo(n, v) _a = [] i = j = nil trap(:break) do (0...n).each do i = it (0...n).each do j = it if i + j == v then throw :break if cond _a << [i, j] end end end end end ``` (of course we need to introduce new escape handling mechanism) ---------------------------------------- Feature #22132: Scala-like for comprehensions https://bugs.ruby-lang.org/issues/22132#change-117843 * 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) } ``` For example, for comprehensions can be used with [a parser-combinator library](https://github.com/shugo/packrat_parser): ```ruby class SimpleCalcParser < PackratParser def additive for x in multitive << term("+"), y in additive then x + y end | for x in multitive << term("-"), y in additive then x - y end | multitive end ``` Here `<<` is equivalent to Scala's `<~`: it runs both parsers but keeps only the left operand's value. ## 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: A for comprehension currently scopes the loop variables unlike `for ... do`. Should it leak them? * Guards desugar to `filter`, not a lazy `withFilter` as in Scala. So in the strict case, each guard builds one intermediate array; users who want fusion can add `.lazy`. Is `filter` acceptable, or should we add a `with_filter`-like method? -- https://bugs.ruby-lang.org/