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Christopher Diggins

Dr. Dobb's Bloggers

Sequences in Clojure and in Heron

December 17, 2009

In Heron the fundamental data type is the sequence "Seq", which is a abstract representation of a collection. In turns out that this idea is very similar to the Clojure approach to sequences.

The fundamental good idea that Clojure and Heron share regarding collections, is that programmers should work with an abstract representation of a collection as much as possible.  

In Heron this is the Seq type, which can be an abstract representation of either a lazily evaluated list (e.g. Iterator) or a concrete type (e.g List). It has only two member functions: "ToList()" and "ToIterator()".  There are three operators which will accepts a Seq type: mapeach, select, and accumulate. There is also the ubiquitous "foreach" construct which takes a Seq as input. 

In Clojure the basic collection type is called an ISeq, and is very similar in design to the Heron sequence, once you get over the superficial differences. I won't talk to much about it because the documentation on Clojure sequences is a must read.  

I didn't actually know about Clojure sequences when I designed the Heron sequence concept, it was instead based on how the C# IEnumerable works, but with better language support for fundamental list operations. 

So what does this all mean for programmers? In Heron the sequence operators are convenient, and have counterparts in C# as libraries (e.g. LINQ). The difference for Heron from LINQ is that the compilers (and metaprograms) can leverage specific assumptions and knowledge about these operators to optimize the dickens out of list code. For an example see this article about optimizing Haskell via Map fusions.

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