accumulate.swift

// accumulate (scanl)
public func accumulate<S : SequenceType, T>
  (sequence: S, initial: T, combine: (T, S.Generator.Element)->T)
  -> SequenceOf<T>
{
  return SequenceOf {_ -> GeneratorOf<T> in
    var g = sequence.generate()
    var value:T? = initial
    return GeneratorOf {
      switch value {
      case .None: return nil
      case .Some(let x):
        value = g.next().map { combine(x, $0) }
        return x
      }
    }
  }
}

// scanl1
public func accumulate<S : SequenceType>
  (sequence: S, combine: (S.Generator.Element, S.Generator.Element)->S.Generator.Element)
  -> SequenceOf<S.Generator.Element>
{
  return SequenceOf {_ -> GeneratorOf<S.Generator.Element> in
    var g = sequence.generate()
    var value = g.next()
    return GeneratorOf {
      switch value {
      case .None: return nil
      case .Some(let x):
        value = g.next().map { combine(x, $0) }
        return x
      }
    }
  }
}


// accumulate([1,1,1,1], 0, +).array //=> [0,1,2,3,4]
// accumulate([1,1,1,1], 0, -).array //=> [0,-1,-2,-3,-4]
// accumulate([1], 0, -).array       //=> [0,-1]

// accumulate([1,1,1,1], +).array   //=> [1,2,3,4]
// accumulate([1,2,3,4,5], +).array //=> [1,3,6,10,15]
// accumulate([1,2,3,4,5], *).array //=> [1,2,6,24,120]