[Haskell-cafe] nice simple problem for someone struggling....

Nicholls, Mark Nicholls.Mark at mtvne.com
Fri Dec 21 11:50:23 EST 2007

Now I have....

module Main where

data SquareType numberType = Num numberType => SquareConstructor

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'	


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

"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:jules at jellybean.co.uk] 
Sent: 21 December 2007 15:33
To: Nicholls, Mark
Cc: haskell-cafe at haskell.org
Subject: Re: [Haskell-cafe] nice simple problem for someone

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...


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