Re: [Haskell-cafe] problem with instance method

No, obj is a method of the Objects class. you've already declared it in the instance of Objects Object your code works just fine here. adding:

mycar = Car Blue

o:: Object Car Integer o = obj mycar 4

ghci says... *Objects> :t obj obj :: (Objects o t i) => t -> i -> o t i *Objects> o Obj (Car Blue) 4 But I hope you read my last email in the other thread you started... Tom On Thu, Feb 3, 2011 at 10:15 AM, Ivan Lazar Miljenovic wrote:

On 3 February 2011 21:09, Patrick Browne wrote:

...

Hi, I am studying type classes using examples from the literature [1]. The attached code is a formalization of basic object oriented ideas. The particular approach seems to be built on the concepts of: thing, object, and identifier. I have no intension to implement anything or significantly change the code below. Rather, I am trying to understand the code as it stands. I include a number of test runs which seem OK, but I cannot get the *obj* function to work.

obj :: t -> i -> o t i obj t i = Obj t i

The type signature says "Given any `t' and any `i', this will return any `o t i' (for some unspecified type `o')".  However, your actual implementation uses a specific data-type, namely Object for `o'. Change the type signature to be "obj :: t -> i -> Object t i".

-- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe