# [Haskell-cafe] automatically deriving Map and Filter on datatypes etc.

Thomas Davie tom.davie at gmail.com
Thu Jun 5 04:39:16 EDT 2008

```Even deriving an instance of Functor seems rather implausable, what
should it do for

data Wierd a b = Nil | A a (Wierd a b) | B b (Wierd a b)

Should fmap's function argument operate on 'a's, 'b's, or both?

Bob

On 5 Jun 2008, at 10:28, Miguel Mitrofanov wrote:

> Well, it's certainly not possible for "filter", at least, not
> without additional hints to the compiler. For example, consider this
> type:
>
> data Weird a = A | B a (Weird a) (Weird a)
>
> filter p A = A
> filter p (B x w1 w2) | p x = B x (filter p w1) (filter p w2)
>                     | otherwise = ?????
>
> On 5 Jun 2008, at 12:03, Cetin Sert wrote:
>
>> Hi ^_^,
>>
>> Let's say we have the following data type and functions:
>> data Tab a =      (:↺:)
>>
>>           |     a :↓:   Tab a
>>           | Tab a :↙↘: (Tab a,Tab a)
>>
>> map f (:↺:)          = (:↺:)
>> map f (a :↓: t)      = f a :↓: map f t
>> map f (h :↙↘: (l,r)) = map f h :↙↘: (map f l, map f r)
>>
>>
>> filter p (:↺:)          = (:↺:)
>> filter p (a :↓: t)      | p a       = filter p t
>>                        | otherwise = a :↓: filter p t
>> filter p (h :↙↘: (l,r)) = filter p h :↙↘: (filter p l,
>> filter p r)
>>
>> is it possible to automatically derive map and filter?
>> data Tab a =      (:↺:)
>>           |     a :↓:   Tab a
>>           | Tab a :↙↘: (Tab a,Tab a)
>>           deriving (Eq, Show, Read, Map, Filter)
>>
>> If not, do you think it might be nice to have something like this
>> in the future?
>>
>> Best Regards,
>> Cetin Sert
>> _______________________________________________