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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