I fixed the issue by installing GHC 6.12, and the latest version of haskell-platform. It worries me that when building against two different versions of the same library ghc more or less fails silently. Near as I can tell, it attempts to link against a new version of mtl, records the symbol for MonadIO, then links in happstack, notices that there's no definition for the MonadIO symbol referenced in happstack, and silently ignore the class instance defined in happstack. This seems depressingly non-deterministic, and makes me wonder how hard it would be to create some sort of check when linking to see if two libraries define/reference different classes that have the same name and at least print some sort of warning about it. The fact that mtl defines MonadIO, but a different version than is referenced in happstack should have been enough to at least generate a warning. Of course, for all I know there is an option for GHC that provides such a warning, but I've no idea what it is and it probably should be enabled by default anyway.<br>
<br>At this point I think anything remotely web-devel related has been covered, so I think I'll take further discussion of this apparent disconnect between class versions to either the #haskell channel, or maybe the haskell-cafe mailing list.<br>
<br clear="all">-R. Kyle Murphy<br>--<br>Curiosity was framed, Ignorance killed the cat.<br>
<br><br><div class="gmail_quote">On Tue, Apr 20, 2010 at 15:24, Jeremy Shaw <span dir="ltr"><<a href="mailto:jeremy@n-heptane.com">jeremy@n-heptane.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div style=""><div class="im"><div><div>On Apr 20, 2010, at 9:02 AM, Kyle Murphy wrote:</div><blockquote type="cite"><font color="#000000"><br></font>It also seems strange that this just started happening recently. I've upgraded a number of packages recently, but I can't say for sure what I changed since the last time I was able to compile something based on happstack successfully. Is it possible that some change with mtl or some other package is causing this error?<br>
</blockquote></div><br></div><div>I am guessing that some of the libraries you depend on are compiled against one version of mtl, and some are compiled against another version. This is not an error in itself. However, you will run into issues similar to what you have described when GHC tries to use version of MonadIO from different versions of the mtl library.</div>
<div><br></div><div>Does "ghc-pkg describe mtl" show more than one version of mtl installed? What happens if you try to unregister the older version? </div><div><br></div><div>This seems like an issues with the state of your installed system, and not something happstack specific..</div>
<div><br></div><font color="#888888"><div>- jeremy</div></font></div></blockquote></div><br>