Hi, I'm stuck at page 151 of Real World Haskell and hoping that perhaps some of you can give me a hand here... The code that is giving me trouble is below. data JValue = JString String | JNumber Double | JBool Bool | JNull | JObject [(String, JValue)] | JArray [JValue] deriving (Eq, Ord, Show) type JSONError = String class JSON a where toJValue :: a -> JValue fromJValue :: JValue -> Either JSONError a instance JSON JValue where toJValue = id fromJValue = Right instance JSON Bool where toJValue = JBool fromJValue (JBool b) = Right b fromJValue _ = Left "not a JSON boolean" I don't understand how the JSON typeclass is defined, in particular the fromJValue definition. For instance, when defining the instance for Bool types, then I understand that both functions (toJValue and fromJValue) will be called upon when we supply a Bool type, but then the (JBool b) type in function fromJValue doesn't match.... toJValue is no problem, but I cannot understand how fromJValue is supposed to work, and the comments in the online book (http://book.realworldhaskell.org/read/using-typeclasses.html) don't help with this either. *Main> :load ch6 [1 of 1] Compiling Main ( ch6.hs, interpreted ) Ok, modules loaded: Main. *Main> toJValue False JBool False *Main> :type it it :: JValue *Main> fromJValue False <interactive>:1:11: Couldn't match expected type `JValue' against inferred type `Bool' In the first argument of `fromJValue', namely `False' In the expression: fromJValue False In the definition of `it': it = fromJValue False *Main> fromJValue (JBool False) <interactive>:1:0: Ambiguous type variable `a' in the constraint: `JSON a' arising from a use of `fromJValue' at <interactive>:1:0-23 Probable fix: add a type signature that fixes these type variable(s) *Main> Any pointers? Thanks a lot, Ángel de Vicente -- http://www.iac.es/galeria/angelv/ High Performance Computing Support PostDoc Instituto de Astrofísica de Canarias --------------------------------------------------------------------------------------------- ADVERTENCIA: Sobre la privacidad y cumplimiento de la Ley de Protección de Datos, acceda a http://www.iac.es/disclaimer.php WARNING: For more information on privacy and fulfilment of the Law concerning the Protection of Data, consult http://www.iac.es/disclaimer.php?lang=en