From: Matthew Flatt <firstname.lastname@example.org> To: email@example.com Subject: [plt-scheme] 352.7 Date: Sat, 14 Oct 2006 06:11:46 0800 MzScheme and MrEd are now version 352.7 in the SVN repository trunk. The main change is the addition of delimited and composable continuations (see below). Other changes: * Changed `make-file-or-directory-link’ to not expand its first argument; instead, the path is written verbatim as a link. * Changed `make-security-guard’ to accept an optional guard procedure to control links. The default disallows the creation of links via `make-file-or-directory-link’. * Removed `write-image-to-file’ and `read-image-from-file’, finally. * Changed `call/ec’ so that its argument is no longer applied in tail position. This reverses a change introduced in v350, and it makes "escape continuations" more like prompts than continuations. * Mac OS X event handling is different internally, but hopefully behaves the same (or slightly better). Matthew Delimited continuations ------------------------ If you’re not familiar with delimited continuations, I recommend starting with "Control Delimiters and Their Hierarchies" Sitaram and Felleisen Lisp and Symbolic Computation, 1990 (available from http://www.ccs.neu.edu/scheme/pubs/ ) MzScheme’s support for prompts and composable continuations most closely resembles Dorai Sitaram’s tagged `%’ and `fcontrol’ operators; see "Handling Control", PLDI’93, or see Dorai’s dissertation. The key feature of Dorai’s `%’ is that it accepts a procedure to handle continuation escape to the prompt. A programmer who installs a prompt can thus choose whether the prompt can be removed. At the same time, having `call/cc’ means that callees can choose whether to keep the prompt, if removing it is an option. Consequently, the "control.ss" library in MzLib offers `prompt’, `control’, `shift’, `prompt0’ (a.k.a. `set’), `control0’ (a.k.a. `cupto’), etc. operators that all work together in a reasonable and useful way. MzScheme’s composable continuations capture and invoke `dynamic-wind’ thunks. To give programmers more flexibility with respect to escapes when using composable continuations, the `fcontrol’ operator is actually split into separate capture and abort operations. Meanwhile, `call/cc’ essentially belongs to the `dynamic-wind’ subsystem, since it provides a way to run a subset of the `dynamic-wind’ thunks since the shared (if any) prompt between the current and captured continuations. Composable continuations also capture continuation marks, which provides a dynamic binding mechanism analogous to that of Kiselyov, Shan, and Sabry (ICFP’06). Summary of the relevant procedures in MzScheme: Basic control: call-with-continuation-prompt : `%’ call-with-composable-continuation \ abort-current-continuation / together, `fcontrol’ make-continuation-prompt-tag : new type for prompt tags (instead of just using symbols) default-continuation-prompt-tag : default tag (used, e.g., by `call/cc’ when no specific tag is supplied) Dynamic wind: dynamic-wind call/cc : accepts an optional prompt tag Continuation marks: with-continuation-mark continuation-marks : accepts an optional prompt tag Use `(lib "control.ss")’ for more readable traditional forms, such as `prompt’, `control’, `shift’, and `reset’. For now, parameters do not work well with composable continuations, because all parameter settings are grouped into a "parameterization" (that is attached to a continuation through a single continuation mark). In other words, there’s no such thing as a delimited parameterization. This mismatch is hopefully temporary.