Ok so I have a problem that was described here ( http://permalink.gmane.org/gmane.comp.lang.haskell.web/1431) in regards to returning a "Ambiguous type variable `a0' in the constraint error" when compiling. Originally I thought it was due to the way I was coding that part of the code (or to be more accurate the code specifically for creating the so called widgets with addHTML/addWidget,addHamlet). So I copied the example code given here exactly (http://www.yesodweb.com/book/example-widgets). Compiling this worked fine, so at the next point I changed the definition of getRootR to getRootR = defaultLayout $ wrapper $ do setTitle "Polymorphic Hamlet" addHtml [$hamlet|<p>I was added with addHtml|] addHamlet [$hamlet|<p>I was added with addHamlet|] addWidget [$hamlet|<p>I was added with addWidget|] and then to getRootR = defaultLayout $ do setTitle "Polymorphic Hamlet" addHtml [$hamlet|<p>I was added with addHtml|] addHamlet [$hamlet|<p>I was added with addHamlet|] addWidget [$hamlet|<p>I was added with addWidget|] Both times compiled fine, so the issue wasn't what I originally thought that it was (as described in http://permalink.gmane.org/gmane.comp.lang.haskell.web/1431). The problem is, that when I use the above example code in my WebServer, I get this Ambigious type error when compiling (even though I have set up the route the exact same way). Unfortunatley I can't provide the code for my webserver, since its commercially owned (and it would be pointless because its just renamed variables, but otherwise its the same), so does anyone have any ideas what could possibly cause such an error (such as a missing/extra import or some package or something), or possibly some missing instances? Also sorry for creating another mailing list, but its a different issue then what I thought it was originally (and I also wanted to put it on haskell-cafe since its a more general issue)