On Dec 21, 2007 12:47 PM, Nicholls, Mark <Nicholls.Mark@mtvne.com> wrote:
Let me resend the code…as it stands….
*module* Main *where*
*data* SquareType numberType = Num numberType => SquareConstructor numberType
*class* ShapeInterface shape *where*
area :: Num numberType => shape*->*numberType
*data* ShapeType = forall a. ShapeInterface a => ShapeType a
*instance* (Num a) => ShapeInterface (SquareType a) *where*
area (SquareConstructor side) = side * side
Part of the problem is that GHC's error messages have to account for a lot of complex typing extensions you aren't using, so they aren't clear. I'll try to explain what's going wrong. If you look at the function, area (SquareConstructor side) = side * side in isolation (that is, not as part of the class instance), it has the type "forall a. Num a => SquareConstructor a -> a". The function in the class declaration has type "forall a b. (ShapeInterface a, Num b) => a -> b". The problem is that a and b are independent variables, but the instance declaration for SquareType a requires them to be related. I'm not sure which way you were trying to parameterize things, but here are some possibilities: (1) If every shape type is parameteric, then you can make ShapeInterface a class of type constructors. class ShapeInterface shape where area :: Num a => shape a -> a instance ShapeInterface SquareType where area (SquareConstructor side) = side * side (2) If only some shape types are parametric, you can use a multi-parameter type class to express a relationship between the shape type and the area type: class ShapeInterface shape area where area :: shape -> area instance (Num a) => ShapeInterface (SquareType a) a where area (SquareConstructor side) = side * side (3) If you only need to be parameteric over some subset of numeric types, you can use conversion functions: class ShapeIterface shape where area :: shape -> Rational class (Real a) => ShapeInterface (SquareType a) where area (SquareConstructor side) = toRational (side * side) (Equivalently, you can replace Rational and Real with Integer and Integral.) It may be that none of these are what you want. There are other, more complex possibilities, but I expect they're overkill. -- Dave Menendez <dave@zednenem.com> <http://www.eyrie.org/~zednenem/>