Now I have....
module Main where
data SquareType numberType = Num numberType => SquareConstructor
numberType
data RectangleType = RectangleConstructor Int Int
class ShapeInterface shape where
area :: shape->Int
data ShapeType = forall a. ShapeInterface a => ShapeType a
instance ShapeInterface (SquareType numberType) where
area (SquareConstructor sideLength) = sideLength * sideLength
main = do
putStrLn (show (area (SquareConstructor 4)))
name <- getLine
putStrLn ""
but get the errors....
In the expression: sideLength * sideLength In the definition of `area':
area (SquareConstructor sideLength) = sideLength * sideLength In the
definition for method `area'
And
Couldn't match expected type `Int' against inferred type `numberType'
`numberType' is a rigid type variable bound by
But to be fair....I almost understand the errors....which is not bad for
me.....surely
"class ShapeInterface shape where
area :: shape->Int"
now looks dubious....I want it to be something like
"class ShapeInterface shape where
area :: Num numberType => shape->Int" ?
but my instance declaration still complains with the errors above and I
now get an error in the class declaration
`numberType1' is a rigid type variable bound by....
It's slightly doing my head in....and reminds me of trying to learn C++
once....not a pleasant experience....though I did eventually
succeed....to a degree.
-----Original Message-----
From: Jules Bean [mailto:[EMAIL PROTECTED]
Sent: 21 December 2007 15:33
To: Nicholls, Mark
Cc: haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] nice simple problem for someone
struggling....
Nicholls, Mark wrote:
> *instance* ShapeInterface SquareType *where*
>
> area (SquareConstructor sideLength) = sideLength * sideLength
> *data* SquareType a = Num a => SquareConstructor a
Now you have changed your type from SquareType to SquareType a, you need
to change the instance to:
instance ShapeInterface (SquareType a) where...
Jules
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