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

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 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