Calculate PI using DataWeave

pi.dwl

/**
 * This function returns a stream. We first populate the stream with a head value,
 * and a lazy tail. 
 * In this case, we tell the engine the tail is a function call. 
 * That function call will be not be executed UNTIL the value of that call is needed.
 */
fun leibnizTerm(n: Number = 0): Array<Number> = 
  [4 / (n + 1) - 4 / (n + 3) ~ leibnizTerm(n + 4)]
// ^^^^^^^^^^^^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^^^^^
//           head                  lazy tail


/**
 * Now we have the sequence generator, the only thing left is select 
 * the values and aggregate them. We are going to pick a slice of the
 * infinite sequence of leibnizTerm using a range selector.
 */
fun pi(terms: Number = 1E4): Number = do {
  var seriesSubset = leibnizTerm()[0 to terms]
  //                 ^^^^^^^^^^^  ^^^^^^^^^^^^
  //                 series gen   range selector
  ---
  sum( seriesSubset )
} 


---
pi(1E4)

// Outputs: 3.1415426585893202