[Haskell-cafe] problem with instance method

Ivan Lazar Miljenovic ivan.miljenovic at gmail.com
Thu Feb 3 11:15:42 CET 2011

On 3 February 2011 21:09, Patrick Browne <patrick.browne at dit.ie> 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 at gmail.com

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