Implementing a String Augmentation Function using FP Techniques

// If successful, will return an augmented copy of the input string
// If the returned value equals the input, that means there is no more information to extract out of the input
declare const addMore: (input: string) => T.Effect< 
     MyContext, 
     MyError, 
     string
 >;

const keepAddingUntilNothingNew = (
   initial: string,
   maxIterations?: number,
): T.Effect< 
     MyContext, 
     MyError, 
     string
 > => 
     T.gen(function* ($) {
         let previous; 
         let latest = initial; 
         let iterationNum = 0; 
  
         do { 
             iterationNum++; 
  
             previous = latest; 
             latest = yield* $(addMore(latest)); 
         } while ( 
             // Continue as long as the iteration gave us new results 
             !EQ.equals(previous, latest) 
  
             // Continue forever if no max iterations is provided, 
             // or continue as long as we haven't hit that maximum 
             && (P.isNullable(maxIterations) || iterationNum < maxIterations) 
         ); 
  
         return latest; 
     });