The compiler defaults the kind of 'quality' (i.e. the first argument of QUALITIES) to *, not being able to infer it from the class definition itself (and other definitions that it references).
Since you want it to have kind * -> *, you should enable KindSignatures and add an annotation, or otherwise disambiguate the kind. This behaviour follows the Haskell Report. The change from previous versions of GHC is documented here: http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/release-7-4-1.html#id3015054 Roman * Patrick Browne <patrick.bro...@dit.ie> [2013-07-09 12:45:19+0100] > Hi, > The code [1] below compiles and runs with GHCi version 7.0.4. > I get one warning and an error message with GHCi version 7.6.1. > 1) Warning -XDatatypeContexts is deprecated. Unless there are > propagation effects, this is well explained. > 2) foom-1.hs:65:15: > `quality' is applied to too many type arguments > In the type `quality entity -> agent -> IO Observation' > In the class declaration for `OBSERVERS' > Failed, modules loaded: none. > I do not understand the error message from 7.6.1. > I am not too interested actually fixing it, I just want to understand > it. > Thanks, > Pat > [1]The code is from: A Functional Ontology of Observation and > Measurement Werner Kuhn > {-# LANGUAGE DatatypeContexts,MultiParamTypeClasses #-} > module ENDURANTS where > import System.Time > type Id = String > type Position = Integer > type Moisture = Float > type Celsius = String > type Heat = Float > data WeatherStation = WeatherStation Id Position deriving Show > data Value = Boolean Bool | Count Int | Measure Float | Category String > deriving Show > data Observation = Observation Value Position ClockTime deriving Show > data AmountOfAir = AmountOfAir Heat Moisture deriving Show > muensterAir = AmountOfAir 10.0 70.0 > class ENDURANTS endurant where > > -- must add instances all down the hierarchy for each instance > instance ENDURANTS WeatherStation where > instance ENDURANTS AmountOfAir where > class ENDURANTS physicalEndurant => PHYSICAL_ENDURANTS physicalEndurant > where > instance PHYSICAL_ENDURANTS WeatherStation where > instance PHYSICAL_ENDURANTS AmountOfAir where > class PHYSICAL_ENDURANTS amountOfMatter => AMOUNTS_OF_MATTER > amountOfMatter where > instance AMOUNTS_OF_MATTER WeatherStation where > class PHYSICAL_ENDURANTS physicalObject => PHYSICAL_OBJECTS > physicalObject where > instance PHYSICAL_OBJECTS WeatherStation where > class PHYSICAL_OBJECTS apo => APOS apo where > getPosition :: apo -> Position > instance APOS WeatherStation where > getPosition (WeatherStation iD pos) = pos + 10 > > -- a data type declaration and data type constructor. > data PHYSICAL_ENDURANTS physicalEndurant => Temperature > physicalEndurant = Temperature physicalEndurant deriving Show > -- Qualities the class of all quality types (= properties) is a > constructor class > -- its constructors can be applied to endurants, perdurants, qualities > or abstracts > class QUALITIES quality entity > instance QUALITIES Temperature AmountOfAir > class (APOS agent, QUALITIES quality entity) => OBSERVERS agent quality > entity where > observe :: quality entity -> agent -> IO Observation > express :: quality entity -> agent -> Value > observe quale agent = do > clockTime <- getClockTime > return (Observation (express quale agent) > (getPosition agent) clockTime) > instance OBSERVERS WeatherStation Temperature AmountOfAir where > express (Temperature (AmountOfAir heat moisture)) weatherStation = > Measure heat > {- > -- running the following > express (Temperature (AmountOfAir 40 20)) (WeatherStation "rr" 6) > -- Gives > Measure 40.0 Measure 40.0 > -- We can get the type: Value > :t express (Temperature (AmountOfAir 40 20)) (WeatherStation "rr" 6) > -} > > Tá an teachtaireacht seo scanta ó thaobh ábhar agus víreas ag Seirbhís > Scanta Ríomhphost de chuid Seirbhísí Faisnéise, ITBÁC agus meastar í a > bheith slán. [1]http://www.dit.ie > This message has been scanned for content and viruses by the DIT > Information Services E-Mail Scanning Service, and is believed to be > clean. [2]http://www.dit.ie > > References > > 1. http://www.dit.ie/ > 2. http://www.dit.ie/ > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
