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

I don't know much about this database stuff, but I'm pretty sure you want to *build* the selector, rather than *extracting* it. You should use the Generics metadata to get the string for parsing/printing, but aside from that, you just have the products. On Nov 23, 2016 10:19 PM, "Chris Kahn" wrote: Aaaand you'll get mine twice since I forgot to reply-all the first time :) Sure, so in postgresql-simple there are two classes for automatically generating functions that encode/decode database rows, `FromRow` and `ToRow`. In the Hasql library--another postgres library--the encoders and decoders must be written by hand for each user-defined type. I want to write a class that will automatically generate these. I successfully wrote a `FromRow` class that can generate Hasql's `Row` type, since it's basically identical to what's in postgresql-simple's `FromRow`. But in Hasql the encoder type, Params, is contravariant and encoders are defined like: personEncoder :: Params Person personEncoder = contramap name (value text) <> contramap age (value int) The `value text` part can be determined based on the type information, but it's also expecting a matching selector function. I'm at a total loss for how I could generate something like this. On 11/23/2016 10:06 PM, David Feuer wrote:

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, 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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