Pluggable, Iterative Type Checking for Dynamic Programming Languages Tristan Allwood (
[email protected]) Professor Susan Eisenbach (supervisor) Dr. Sophia Drossopoulou (second marker) University of London Imperial College of Science, Technology and Medicine Department of Computing Copyright c Tristan Allwood, 2006 Project sources available at: /homes/toa02/project/fleece/ June 13, 2006 Abstract Dynamically typed programming languages are great. They can be highly expressive, incredibly flexible, and very powerful. They free a programmer of the chains of needing to explicitly say what classes are allowed in any particular point in the program. They delay dealing with errors until the last possible moment at runtime. They trust the programmer to get the code right. Unfortunately, programmers are not to be trusted to get the code right! So to combat this, programmers started writing tests to exercise their code. The particularly conscientious ones started writing the tests before the code existed. The problem is, tests are still code, and tests would still only point out errors at runtime. Of course, some of these errors would be silly mistakes, e.g. misspelled local variables; some would be slightly more complicated, but a person reading the code could notice them. They are errors that could be detected while a program was being written. Static type systems could help detect these mistakes while the programs were being written. Retrofitting a mandatory static type system onto a dynamically typed programming language would not be a popular action, so instead the idea of optional and then pluggable type systems was raised. Here the type systems can be applied to the language, but they don’t have to be.