Lazy Data Streams

A Lazy Data Stream is a mix between an iterator and a list. A stream can be expanded by lazy calculation of an extra member.

Streams is a key component of the declarative programming engine in ShapeLogic starting from ShapeLogic 1.0.

Using streams to solve mathematical problems

Project Euler is a list of 178 mathematical problems that can be solved by computers. The first 10 mathematical problems have been solved in ShapeLogic using streams.


Lazy Data Streams are used in different functional languages e.g.

  • Haskell
  • Scala
  • Scheme

The advantages of a lazy data stream are

  • That you can use functional techniques on possible infinite streams of data
  • You still have access to previous elements in a calculation by an iterator
  • You only calculate the necessary elements

Examples of lazy infinite data streams

  • Lazy Data Streams to calculate all Fibonacci numbers
  • All even Fibonacci numbers
  • All even Fibonacci numbers greater than 1000


  • last the last element of a stream, set to LAST_UNKNOWN, before it is know. This value can get lower as calculation progress.
  • maxLast manually set max value for last. The real last value might be lower.
  • nullLegalValue if this is false then the stream will end when the first null value is encountered.

How can an calculation of stream end?

  • If null is not a legal value when finding a null
  • If an exception is thrown during a calculation
  • By setting a top maxLast value

How to iterate over a stream

For loop

Stream stream; 
for (int i=0; 
     stream.getLast() != Constants.LAST_UNKNOWN || i <= stream.getLast(); 
        Object = stream.get(i);

While loop

Stream stream; 
while (stream.hasNext())
        Object stream.get(i);

Stream hierarchy

There is a quite extensive hierarchy of streams. Here are the most important:

  • Stream top interface. Mainly just an Iterator.
  • NumberedStream interface. Stream that can be accessed with an index.
  • ListStream interface. Stream that stores result in a List.
  • BaseListCommonStream base class for most implementations.

Links for more info