[Haskell-beginners] How to add a "method" to a record

[martin]

Am 09/10/2014 08:50 PM, schrieb Corentin Dupont:
> If the field "label" can be deduced from "payload", I recommend not to include it in your structure, because that would
> be redundant.
> 
> Here how you could write it:
> 
> data Foo pl = Foo { payload :: pl}
> 
> labelInt :: Foo Int -> String
> labelInt (Foo a) = "Int payload:" ++ (show a)
> 
> labelString :: Foo String -> String
> labelString (Foo a) = "String payload" ++ a
> 
> You are obliged to define two separate label function, because "Foo Int" and "Foo String" are two completly separate types.

This is exactly my problem: Someone will use this type an define the type of pl. How can I know what type she'll use?
What I'd like to express is that whoever creates a concrete type should also provide the proper label function.

> 
>         On Wed, Sep 10, 2014 at 2:06 PM, martin <martin.drautzburg at web.de> wrote:
> 
>             Hello all
> 
>             if I have a record like
> 
>                     data Foo pl = Foo {
>                                 label :: String,
>                                 payload :: pl
>                             }
> 
>             how can I create a similar type where I can populate label so it is not a plain string, but a function which
>             operates on
>             payload? Something like
> 
>                     label (Foo pl) = show pl
>