haskell - Function Argument that might or might not instantiate a TypeClass? -

this i'm trying make handler -

getuserstudentsr :: userid -> handler typedcontent getuserstudentsr userid =        getstudententitiesforcoach userid        >>= returntypedcontent . map touserstudentresponse  

where student persistent entity (details not important) ,

getstudententitiesforcoach :: userid -> handlert app io [entity student] getstudententitiesforcoach coachid =      rundb $ selectlist [studentprimarycoachid ==. just(coachid)] []   data userstudentresponse = studentresponse (key student) student  instance tojson userstudentresponse     tojson (studentresponse studentid student) =            object                   [                       "login" .= studentlogin student                       , "studentid" .= studentid                       , "firstname" .= studentfirstname student                       , "lastname" .= studentlastname student                    ]  touserstudentresponse :: (entity student) -> userstudentresponse touserstudentresponse (entity studentid student)      = studentresponse studentid student 

and

returntypedcontent x = selectrep $                            providejson x 

now obviously, doesn't compile unless userstudentresponse instantiates tojson , provides implementation tojson. however, want make returntypedcontent function generic - -

returntypedcontent x = selectrep $                           -- if tojson x --                              providejson x                          -- if tohtml x -- -- note not either or                             providerep $ return $ tohtml $ 

i want this, returntypedcontent can expanded provide returns kinds of contenttypes , based on whether data type used in handler instantiates typeclasses (such tojson), have different things provided for.

is possible without going template haskell?

you use (enable gadts):

data wrapper     json :: tojson => -> wrapper     html :: tohtml => -> wrapper 

and can pattern match:

returntypedcontent (json x) = selectrep (providejson x) returntypedcontent (html x) = selectrep (providerep $ return $ tohtml x) 

you have explicitly wrap data, shouldn't problematic.