Thursday, May 06, 2004

A summary of Avi Bryants Seaside talk. In the summary there is a section about 'ensure' and how it works in the presence of continuations. It mentions that 'Scheme has continuations. Common Lisp has no continuations because it has ensure'. Scheme has a form of 'ensure'. It is called 'dynamic-wind' and allows code to be executed before entry and after exit of the dynamic-wind block. It works in the presence of continuations in that whenever the code block is 're-entered' by a continuation the 'entry' block is executed again, and the 'exit' block executed on leaving the code block (whether leaving naturally or via escaping through a continuation). Kent Pitman has an article on 'UNWIND-PROTECT vs Continuations'. UNWIND-PROTECT is the Common Lisp equivalent of dynamic-wind. It allows a section of code to be executed whenever a block is exited. In that article Kent points out that dynamic-wind really solves a different problem than UNWIND-PROTECT and explains why you cannot implement the latter in the prescence of continuations easily. Dorai Sitaram wrote a paper 'Unwind-protect in portable Scheme' that outlines some ways that you can write UNWIND-PROTECT. 10:39:20 AM

From naseby+ruby+stuff: A language that targets the .NET CLR, Nemerle. It has macros, pattern matching, static and on-demand typing, type inference and functional features. 10:22:08 AM