[Haskell-cafe] functional dependencies question

[Patrick Browne]

Hi,
My understanding of functional dependencies is that they can be used to
ensure that one type depends on another type.
For example, the type of location depends could on the type of the
object at that location.
Consider two models
1) The location of an aircraft landing should have location of the type
AirportCode,
2) The location of a car's destination should have a type
CartesianCoordinate.

AirportCodes should not be used as locations in 2) and
CartesianCoordinates should not be used as locations in 1).
Haskell class, instances, and fundeps were used to represent the above
requirements. I generalized the requirement a bit, testing on a variety
of types (below).

Obviously, some cases are ambigous and/or conflicting and the Haskell
compiler correctly flags this.


Why do some cases such as 1) fail to run even if they are the only
instantiation.

Regards,
Pat



{-
C:\GHC\ghc-6.8.3\bin\ghci.exe  -XMultiParamTypeClasses
-XFunctionalDependencies -fglasgow-exts -fallow-undecidable-instances
-}

class LocatedAt object location | object -> location where
 spatialLocation :: object -> location


-- 1) Compiles but does not run
instance LocatedAt  Int  String  where
 spatialLocation(1)="home"

-- 2) Compiles and runs OK
instance LocatedAt String Int where
 spatialLocation("home")=1

-- 3) Compiles and runs  OK, but obviously not in conjunction with 2
instance LocatedAt  String  String  where
  spatialLocation("home")="home"

-- 4) Compiles and runs OK
instance LocatedAt  Bool  String  where
  spatialLocation(True)="home"


-- 5) Compiles and runs OK, but obviously not in conjunction with 2
instance LocatedAt    String  Bool where
    spatialLocation("home") = True

-- 6) Not OK
instance LocatedAt  Float  String  where
   spatialLocation(2.3)="home"

-- 7) Compiles but does not run
 instance LocatedAt Int Char  where
  spatialLocation(1)='1'


-- 8)  Compiles and runs OK
instance LocatedAt Char Int   where
 spatialLocation('1')=1

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