[Haskell-cafe] Understanding version differences

[Roman Cheplyaka]

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.browne at 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)
>    -}
> 
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