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.