[The Creation of a Constraint-based Programming Language]. I had to paraphrase the title to fit the maximum length, it's really:
The Definition and Implementation of a Computer Programming Language Based on Constraints, by Guy L. Steele (thesis)
This is a really wonderful piece of writing. What I got out of it was an introduction to constraint-based programming, a gentle tour of growing a language bit by bit, and an amazing demonstration of Lisp techniques for writing interpreters that fit in neatly with their host environment. (And I only read the first half!)
That said, it is book-length and the electronic copy is a scan. The result is 20 megabytes and challenging to print - but well worth the effort. [Lambda the Ultimate]
Steele Jr