<div dir="ltr">Hi all,<div><br></div><div>I think it's no secret that wai-handler-devel is not living up to its full potential. Here's a few of the issues that I know about:</div><div><br></div><div>* Doesn't integrate well with cabal-dev. (Fair warning: I haven't yet started using cabal-dev.)</div>
<div>* It reinterprets the entire application, instead of just the files that changed and their dependencies.</div><div>* It has no knowledge of your cabal file, and so simply tries to use the most recent version of each package available. A few people this week noticed problems running wai-handler-devel since WAI 0.4 was released but is not yet supported by Yesod.</div>
<div><br></div><div>Some of these issues can be solved while still sticking with Hint. However, that will require reimplementing a significant amount of work that already exists in other places. I think our best bet is to re-evaluate entirely what we need, and think about the best way to get there. Let's start by pointing out why "cabal build && ./dist/build/myapp/myapp" isn't good enough:</div>
<div><br></div><div>* Linking an executable is *very* slow.</div><div>* You have to manually kill your application and rerun the above command each time you change a file.</div><div>* If an external file included by Template Haskell, such as a Hamlet template, is modified, cabal does not know to recompile that module.</div>
<div><br></div><div>But the fact is that the simple cabal build approach *does* solve all of the issues we currently have with wai-handler-devel. (Well, at least the three I mention above, feel free to chime in here.) So here's an idea: let's try to build on top of cabal so we can take advantage of its features. As for the points listed above, I'll address them in reverse order.</div>
<div><br></div><div>wai-handler-devel already contains code that determines which "extra" files are actually dependencies for a module. For example, it looks for hamletFile through your source code and, when it finds it, notes that the specified file is a dependency. We can easily write a tool that checks the timestamps on all the files, determines if a file needs to be updated, and modifies timestamps accordingly. For example, if Handler/Root.hs uses hamlet/root.hamlet, and the latter has a newer modification date than the former, the tool would change the former's modification date to the same as the latter.</div>
<div><br></div><div>As far as manually killing applications and rerunning the compile steps, that's also very simple: we simply have a monitor program that sees if source files have been modified since the last build. wai-handler-devel already does this. We can optimize this on Linux with inotify, and the equivalent on other operating systems, but I think it's fair to say that this is a solved problem.</div>
<div><br></div><div>The big question is how to avoid long links. The simple answer is to skip them: we don't actually need an executable, just the object files. We'll then use plugins[1] to hot-load the code... except that plugins seems to be (1) buggy and (2) not cross-platform. We can try out direct-plugins[2] instead, but it seems that it will require us to install the package into GHC's database, which isn't necessarily a good idea. (Perhaps we can ask Dan very nicely to make some API changes to help us here?) The third route is writing the hot-loading code ourselves. Andy Stewart pointed out to me that manatee[3] does this directly against the GHC API.</div>
<div><br></div><div>My guess: our best bet is to ask Dan (direct-plugins's author) to work with us on augmenting his package with the features we need. This can have ramifications outside the realm of wai-handler-devel, such as having a server that hot-loads code for individual applications. (I was thinking of this for Lambda Engine actually.) But before starting down that route, I'd love to hear if others have any opinions on the subject.</div>
<div><br></div><div>Sorry for the long email :)</div><div><br></div><div>Michael</div><div><br></div><div>[1] <a href="http://hackage.haskell.org/package/plugins">http://hackage.haskell.org/package/plugins</a></div><div>
[2] <a href="http://hackage.haskell.org/package/direct-plugins">http://hackage.haskell.org/package/direct-plugins</a></div>
<div>[3] <a href="http://hackage.haskell.org/package/manatee">http://hackage.haskell.org/package/manatee</a></div><meta http-equiv="content-type" content="text/html; charset=utf-8"><meta http-equiv="content-type" content="text/html; charset=utf-8"><meta http-equiv="content-type" content="text/html; charset=utf-8"></div>