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May 2011

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Ambiguous type variable `mo0' in the constraint + Understanding how Widgets work
by Mathew de Detrich 30 Apr '11

30 Apr '11

I am having issues figuring out how exactly widgets work within Yesod, having a look at the example here http://www.yesodweb.com/book/example-widgets <http://www.yesodweb.com/book/example-widgets>There is an example getHomeR = defaultLayout $ do setTitle "Polymorphic Hamlet" addHtml [$hamlet|I was added with addHtml|] addHamlet [$hamlet|I was added with addHamlet|] addWidget [$hamlet|I was added with addWidget|] Now assuming you have a Homer router set up, if you compile the example you get Ambiguous type variable `mo0' in the constraint: (or errors of that form). Asking on haskell irc, I found out this error is due to Haskell not knowing what type Hamlet returns, which is probably the reason for this comment "However, there is a more subtle point as well. While in theory a Hamlet template is polymorphic, this actually depends upon what values are embedded inside it (using ^{...} interpolation). For example, [$hamlet|^{someValue}|] will have the same type as someValue; if someValue is of type Hamlet, then so will the template." I assume that you are meant to have one ^{...} interpolation for every GET route request you are attempting to do, so checking the example code here here http://www.yesodweb.com/book/example-widgets, as I suspected the variable interpolation is used in the root handler and in the form handler (in the case for root handler it goes through a wrapper function). The issue is, when I attempt to use the ^{...} interpolation in my code (in this case I am literally just copying the wrapper function and passing it through with defaultLayout $ wrapper $ do as with the example) I am still getting the ambiguous type variable using this simple example getIndexR = defaultLayout $ wrapper $ do setTitle "Polymorphic Hamlet" addHtml [$hamlet|I was added with addHtml|] addHamlet [$hamlet|I was added with addHamlet|] addWidget [$hamlet|I was added with addWidget|] Am I missing something fundamental here, because I am at odds of what to do at this point (I am using yesod 7.0.3 if that matters). Also is there a non trivial example of how combinating widgets is done to form a combined web page (containing CSS, HTML and JS), i.e. whats mentioned here "The job of a widget is to hold onto these disparate components and apply proper logic for combining different widgets together. This consists of things like taking the first title set and ignoring others, applying nub to the list of external scripts and stylesheets, and simply concatenating head and body content." Thanks

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