<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><br></div><div>Hmmm, my point (perhaps I wasn't clear), is that different effects have different commutability properties. In the case of a file system, you can commute two sequential reads from two different files. This has no effect on the result of the computation, assuming no interference from other programs -- and if there _is_ interference from other programs, then guarantees go out the window, _with or without_ commuting.</div><div><br></div><div>Monads are an insufficient structure to capture the fine semantics of effects. Something more powerful is needed. And in general, the way effects combine and commute is quite complicated and needs to be baked into the effect system, rather than being the responsibility of a lowly developer.</div><div><br></div><div apple-content-edited="true"> <span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div>Regards,</div><div><br></div><div>John A. De Goes</div><div>N-Brain, Inc.</div><div>The Evolution of Collaboration</div><div><br></div><div><a href="http://www.n-brain.net">http://www.n-brain.net</a> | 877-376-2724 x 101</div></div></div></span></div></span></div></span></div></span></div></span> </div><br><div><div>On Aug 13, 2009, at 12:24 PM, Sebastian Sylvan wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><br><br><div class="gmail_quote">On Thu, Aug 13, 2009 at 2:19 PM, John A. De Goes <span dir="ltr"><<a href="mailto:john@n-brain.net">john@n-brain.net</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"> <div style="word-wrap:break-word"><div class="im"><div><blockquote type="cite"><div class="gmail_quote"><div><div>What if you have another program, written in C or something, that monitors a file for changes, and if so changes the contents of another file? Surely to catch that you must mark *all* file system access as "interefering"? Even worse, another program could monitor the state of a file and conditionally disable thet network driver, now file access interferes with network access.</div> </div></div></blockquote></div><div><br></div></div><div>A compiler or runtime system can't know about these kinds of things -- unless perhaps you push the effect system into the operating system (interesting idea). The best you can do is ensure the program itself is correct in the absence of interference from other programs</div> </div></blockquote><div><br></div><div>I think the best you can do is make sure any code which is vulnerable to such interference won't be subject to unsafe transformations (like changing the order of evaluation). </div> <div>So I do think pretty much anything that relies on the outside world needs to go into one big effects "category" so the compiler/runtime will "stay out" and let the programmer explicitly define the ordering of those operations, precisely because the compiler has no way of knowing anything about what kind of assumptions are in effect.</div> </div><br clear="all"><br>-- <br>Sebastian Sylvan<br></blockquote></div><br></body></html>