[Haskell-cafe] Is it possible to get the selector functions when defining a generic class?

David Feuer david.feuer at gmail.com
Thu Nov 24 03:06:32 UTC 2016

Sorry if anyone gets this twice; the first copy somehow went to a
non-existent Google Groups version of haskell-cafe.

GHC.Generics doesn't offer any built-in support for such things. It
*looks* like there *might* be some support in packages built around
generics-sop. When you're working directly with GHC.Generics, the
notion of a record barely even makes sense. A record is seen as simply
a possibly-nested product. For example, ('a','b','c') will look
*approximately* like 'a' :*: ('b'  :*: 'c'). You're generally not
"supposed" to care how large a record you may be dealing with, let
alone what field names it has. May I ask what you're actually trying
to do? Your specific request sounds peculiarly un-generic.

On Wed, Nov 23, 2016 at 9:52 PM,  <chris at kahn.pro> wrote:
> Hey all!
> I'm trying to understand what's going on in GHC.Generics and defining a
> generic class... I understand that there's a `Selector` class and `selName`
> function that can get the name of a selector, but is there a way to access
> the selector function itself? The documentation conveniently avoids examples
> involving records and is otherwise quite barren.
> So if I have a data type like...
> data Person = Person
>     { name :: String
>     , age :: Int
>     } deriving Generic
> instance MyTypeClass Person
> I want my generic implementation of MyTypeClass to be able to access each
> selector function in the record, f :: Person -> String, g :: Person -> Int,
> etc.
> Chris
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