Guaranteed break with

[edemaine]

This seems like a useful construction, analogous to Array.prototype.find:

result := for x of y when condition(x)
  break with x

The generated code

let results;results=[];for (const x of y) {if (!condition(x)) continue;
  results = x;break
};const result =results

sets results = []. It would type better without that. We can detect this by seeing that the loop always exits.

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Related, I wonder if there should be a reducer for this, called for find or for first. It's similar to for some but returning the first item instead of a boolean.