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.
Alternatively, if you want only one label function, use a type sum and pattern match on it:

data T = TS String | TI Int
data Foo2 = Foo2 { payload2 :: T}

label :: Foo2 -> String
label (Foo2 (TI a)) = "Int payload:" ++ (show a)
label (Foo2 (TS a)) = "String payload:" ++ a



On Wed, Sep 10, 2014 at 8:30 PM, Julian Birch <julian.birch@gmail.com> wrote:
Bear in mind you can just create functions at the top level that operate on your data structure.  You only need the function to be a member of the record if the function itself changes, which is relatively rare.

Say you need a Foo and a Bar, and they both have labels, implemented in different ways, then you can use a typeclass to achieve your goals.


On Wednesday, September 10, 2014, David McBride <toad3k@gmail.com> wrote:
Is this what you are looking for?

data Foo pl = Foo {
  label :: pl -> String,
  payload :: pl
}

On Wed, Sep 10, 2014 at 2:06 PM, martin <martin.drautzburg@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


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--
Sent from an iPhone, please excuse brevity and typos.

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