Repetitive Reduction Patterns in Lambda Calculus with letrec (Work in Progress)

For the lambda-calculus with letrec we develop an optimisation, which is based on the contraction of a certain class of 'future' (also: virtual) redexes. In the implementation of functional programming languages it is common practice to perform beta-reductions at compile time whenever possible in order to produce code that requires fewer reductions at run time. This is, however, in principle limited to redexes and created redexes that are 'visible' (in the sense that they can be contracted without the need for unsharing), and cannot generally be extended to redexes that are concealed by sharing constructs such as letrec. In the case of recursion, concealed redexes become visible only after unwindings during evaluation, and then have to be contracted time and again. We observe that in some cases such redexes exhibit a certain form of repetitive behaviour at run time. We describe an analysis for identifying binders that give rise to such repetitive reduction patterns, and eliminate them by a sort of predictive contraction. Thereby these binders are lifted out of recursive positions or eliminated altogether, as a result alleviating the amount of beta-reductions required for each recursive iteration. Both our analysis and simplification are suitable to be integrated into existing compilers for functional programming languages as an additional optimisation phase. With this work we hope to contribute to increasing the efficiency of executing programs written in such languages.Comment: In Proceedings TERMGRAPH 2011, arXiv:1102.226

Multi-user publishing in the Web : DReSS, a Document Repository Service Station

Many WWW servers contain information written by several authors. These authors either need an account on the server machine, and special permissions to create information in the server space, or else the Webmaster needs to put the information in that space or allow the server to point to the author's own space. We present DReSS, a system to enable authors to deposit (and update) documents on a WWW server, using standard WWW features only. Authors do not need login permission on the server machine, ftp upload access, or even electronic mail. As the documents live in the WWW server space there is no need for the server to be able to access documents outside its space. Thus, our system will work on even the most securely shielded servers (running in a chroot environment). DReSS consists of a set of CGI-scripts and two small auxiliary programs running on the client machine. It can be used with any (HTML-2.0-capable) WWW browser, and with any WWW server. DReSS does not use special features ..