Deriving projection functions using `generics-sop` - haskell

Question

How would I go about deriving the function
getField :: (Generic a, HasDatatypeInfo a) => Proxy (name :: Symbol) -> a -> b
to project a field from an arbitrary record using a type-level string (Symbol), using the generics-sop library?
This is similar to Retrieving record function in generic SOP, but I have the following problems:
The OP does not explain how to go the last mile to get the signature I desire.
The OP defines complex special-purpose helper types, which I am keen to avoid
The given solution only errors out at runtime, but compile-time matching should be possible, since a type-level DataTypeInfo is provided through the DatatypeInfoOf type family (nice to have, but not necessary).
The lens-sop package also seems to do something similar, but I can't work out how to make it work for me.
I would also prefer a solution that uses the IsProductType typeclass.

Answer 1

I know this is a mess of an answer and not really what you were looking for, but it's the best I can do right now. Note that this works for both product types and sum types where all the constructors have the specified field name.
I think this could likely be simplified somewhat by separating the name lookup from the rest of the product handling. That is: use the datatype info to calculate the field number (as a unary natural), then use that number to dig through the code. Unfortunately, generics-sop doesn't seem to have really wonderful facilities for working with list zipping, so I ended up doing a lot "by hand".
{-# language EmptyCase, GADTs, TypeFamilies, DataKinds, TypeOperators, RankNTypes #-}
{-# language UndecidableInstances, UndecidableSuperClasses #-}
{-# language AllowAmbiguousTypes, TypeApplications, MultiParamTypeClasses,
FlexibleContexts, FlexibleInstances, MagicHash, UnboxedTuples, ScopedTypeVariables #-}
{-# language ConstraintKinds #-}
{-# OPTIONS_GHC -Wall #-}
module Data.Proj where
import Data.Kind (Type, Constraint)
import Generics.SOP
import Generics.SOP.Type.Metadata as GST
import GHC.TypeLits
import Data.Type.Equality (type (==))
-- This is what you were looking for, but slightly more flexible.
genericPrj :: forall s b a.
( Generic a
, HasFieldNS s b (GetConstructorInfos (DatatypeInfoOf a)) (Code a))
=> a -> b
genericPrj a = case genericPrj# #s a of (# b #) -> b
-- This version lets you force the *extraction* of a field without
-- forcing the field itself.
genericPrj# :: forall s b a.
( Generic a
, HasFieldNS s b (GetConstructorInfos (DatatypeInfoOf a)) (Code a))
=> a -> (# b #)
genericPrj# a = case from a of
SOP xs -> extraction #s #b #(GetConstructorInfos (DatatypeInfoOf a)) #(Code a) xs
-- | Extract info about the constructor(s) from 'GST.DatatypeInfo'.
type family GetConstructorInfos (inf :: GST.DatatypeInfo) :: [GST.ConstructorInfo] where
GetConstructorInfos ('GST.ADT _ _ infos _) = infos
GetConstructorInfos ('GST.Newtype _ _ info) = '[info]
class HasFieldNS (s :: Symbol) b (cis :: [GST.ConstructorInfo]) (code :: [[Type]]) where
extraction :: NS (NP I) code -> (# b #)
instance HasFieldNS s b cis '[] where
extraction x = case x of
instance (HasFieldNP' s b r c, HasFieldNS s b cis cs, rec ~ 'GST.Record q r, VerifyRecord rec)
=> HasFieldNS s b (rec ': cis) (c ': cs) where
extraction (Z x) = extractIt #s #b #rec #c x
extraction (S x) = extraction #s #b #cis #cs x
type family VerifyRecord rec :: Constraint where
VerifyRecord ('GST.Record _ _) = ()
VerifyRecord _ = TypeError ('Text "Constructor is not in record form.")
-- | Given info about a constructor, a list of its field types, and the name and
-- type of a field, produce an extraction function.
class HasFieldNP (s :: Symbol) b (ci :: GST.ConstructorInfo) (fields :: [Type]) where
extractIt :: NP I fields -> (# b #)
instance (HasFieldNP' s b fi fields, ci ~ 'GST.Record _cn fi)
=> HasFieldNP s b ci fields where
extractIt = extractIt' #s #_ #fi
class HasFieldNP' (s :: Symbol) b (fi :: [GST.FieldInfo]) (fields :: [Type]) where
extractIt' :: NP I fields -> (# b #)
class TypeError ('Text "Can't find field " ':<>: 'ShowType s)
=> MissingField (s :: Symbol) where
impossible :: a
instance MissingField s => HasFieldNP' s b fi '[] where
extractIt' = impossible #s ()
instance HasFieldNP'' s b (fi == s) field fis fields =>
HasFieldNP' s b ('GST.FieldInfo fi ': fis) (field ': fields) where
extractIt' = extractIt'' #s #b #(fi == s) #field #fis #fields
class HasFieldNP'' (s :: Symbol) b (match :: Bool) (field :: Type) (fis :: [GST.FieldInfo]) (fields :: [Type]) where
extractIt'' :: NP I (field ': fields) -> (# b #)
instance b ~ field => HasFieldNP'' _s b 'True field fis fields where
extractIt'' (I x :* _) = (# x #)
instance (HasFieldNP' s b fis fields) => HasFieldNP'' s b 'False _field fis fields where
extractIt'' (_ :* fields) = extractIt' #s #b #fis fields
Examples
data Foo
= Foo {blob :: Int, greg :: String}
| Bar {hello :: Char, blob :: Int}
deriveGeneric ''Foo
genericPrj #"blob" (Foo 12 "yo") ===> 12
genericPrj #"blob" (Bar 'x' 5) ===> 5
genericPrj# #"blob" (Bar 'x' 5) ===> (# 5 #)
myAbsurd :: Void -> a
myAbsurd = genericPrj #"whatever"
data Booby a
= Booby {foo :: a}
| Bobby {bar :: a}
deriveGeneric ''Booby
genericPrj #"foo" (Booby 'a')
-- Type error because Bobby has no foo field

Answer 2

As of version 0.1.1.0, records-sop provides this function:
getField :: forall s a b ra. (IsRecord a ra, IsElemOf s b ra) => a -> b
which needs the field name supplied as a type application rather than a proxy, like so:
data Foo = Foo { bar :: Int }
getField #"bar" (Foo 42) === 42
This provides compile-time extraction, although it will still need a bit of casting around to fit in with existing code in my project that manipulates standard generics-sop metadata.
This only works on single-constructor types. #dfeuer's answer also supports sum types.
Thank you #kosmikus, the coauthor of generics-sop and author of records-sop, for implementing this in response to this question!