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POFunction Argument that might or might not instantiate a TypeClass?
 

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    <p>This is what I'm trying to make my Handler look like - </p> <pre><code>getUserStudentsR :: UserId -&gt; Handler TypedContent getUserStudentsR userId = getStudentEntitiesForCoach userId &gt;&gt;= returnTypedContent . map toUserStudentResponse </code></pre> <p>where Student is a Persistent Entity (details mostly not important) and </p> <pre><code>getStudentEntitiesForCoach :: UserId -&gt; HandlerT App IO [Entity Student] getStudentEntitiesForCoach coachId = runDB $ selectList [StudentPrimaryCoachId ==. Just(coachId)] [] data UserStudentResponse = StudentResponse (Key Student) Student instance ToJSON UserStudentResponse where toJSON (StudentResponse studentId student) = object [ "login" .= studentLogin student , "studentId" .= studentId , "firstName" .= studentFirstname student , "lastName" .= studentLastname student ] toUserStudentResponse :: (Entity Student) -&gt; UserStudentResponse toUserStudentResponse (Entity studentId student) = StudentResponse studentId student </code></pre> <p>and</p> <pre><code>returnTypedContent x = selectRep $ do provideJson x </code></pre> <p>Now obviously, this doesn't compile unless UserStudentResponse instantiates ToJSON and provides an implementation for toJSON. However, I want to make the returnTypedContent function generic - something like - </p> <pre><code>returnTypedContent x = selectRep $ do -- if ToJSON x -- provideJSON x -- if ToHTML x -- -- note this is not an either or provideRep $ return $ toHtml $ a </code></pre> <p>I want to do this, so that returnTypedContent can be expanded to provide returns for all kinds of contenttypes and then based on whether the data type used in the handler instantiates certain typeclasses (such as ToJSON), we have different things provided for. </p> <p>Is something like this possible without going into Template Haskell?</p>
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    1. USRoopesh Shenoy
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    1. COI'm not really a yesod user, but what does `selectRep` do? Isn't it sufficient to just create a `ToTypedContent` instance for `UserStudentResponse`?
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      1. USJohn L
    2. COYou can't do that directly (modulo the chance that somebody else comes up with some clever tricks); constraints like `ToJSON [a]` are propositions, not booleans internal to the language. Try writing down the type for `returnTypedContent`. That will hopefully provide some insight as to why you can't do this, and possibly help design a workaround (though I don't immediately know what one would look like).
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      1. USAntal Spector-Zabusky
    3. COwith `-XOverlappingInstances` you can make a "fallback" instance of `ToJSON` and `ToHTML`. You might be able to treat the values you get from the fallback instance specially (they could be `throw SomeVeryUniqueException`, if there's no json/html value that's normally off-limits). You're better off writing a large number of instances with template-haskell I think.
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      1. USaavogt
 

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