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

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 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 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 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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