loop with early return

[IR0NSIGHT]

Feature request:
a for loop with ability to return early/break

Right now there is no such feature (afaik):
each(n,m) executes from start to finish, no option to break early.

In order to use a return early functionality (as is dictated in my case by the existing are-we-fast-yet benchmarks) i have to fall back to "while (run)" ... and mutate run in the loop:

var run = true
while (run) {
   if (...) { run = false}
}

This is much harder to read/understand/verify and invites bugs (mutable state and unnecessary while loop)

so:
each() with break (and continue) would be nice to have.
(any kind of early return would suffice, it doesnt have to be "break")

[Jiří Beneš]

Thanks for the issue!
I'm putting some additional context below:

Existing related work

The ML backend has a Control effect:

effekt/libraries/ml/effekt.effekt

Lines 214 to 217 in 11218d9

 effect Control { 
   def break(): Unit 
   def continue(): Unit 
 } 

and a function each that allows one to use the Control effect and handles it correctly:

effekt/libraries/ml/effekt.effekt

Line 230 in 11218d9

 def each(start: Int, end: Int) { action: (Int) => Unit / Control } = { 

I don't know if we want to port this version of each to the JavaScript backend as well 🤔
Related to #309

Examples

effekt/examples/ml/lib/loops.effekt

Lines 1 to 17 in 11218d9

 def greaterFifteen() = { 
   var total = 0; 
   each(0, 100) { i => 
     total = total + i; 
     if (total > 15) do break() 
     println(i) 
   } 
 } 
  
 def sumPositive() = { 
   var total = 0; 
   each(-10, 10) { i => 
     if (i < 0) do continue() 
     total = total + i 
   } 
   println(total) 
 }