Add to# method

[treeowl]

Suppose I have

fg :: (A -> Maybe A) -> Rep B -> Rep B

Let's say fg sometimes replaces the hello field in a record.

I can define

f :: (A -> Maybe A) -> B -> B
f p b = to (fg p (from b))

Most of the time, that's just dandy. But ... what if B is actually a newtype? The only way to force the application of p is to force the result of f. In hand-written code, we could write a special function for that:

import GHC.Tuple (Solo (..))  -- data Solo a = Solo a, for recent GHC

fS :: (A -> Maybe A) -> B -> Solo B

We can do the same kind of thing generically:

f :: (A -> Maybe A) -> Rep B -> Solo (Rep B)
f p b = case fg p (from b) of
  Solo rb -> Solo (to rb)

But there's an unfortunate effect: the calculation of to itself is suspended, so we hang on to an unneeded Z constructor in that thunk.

I suggest adding a method to Generic:

to# :: Rep a -> (# a #)
-- Custom GHC.Generics-based default goes here, and the default for `to` is replaced by
-- to r = case to# r of (# a #) -> a

and a function

toA :: (Applicative f, Generic a) => Rep a -> f a
toA r | (# a #) <- to# r = pure a

[treeowl]

Note that GHC generics don't have this issue, because for newtypes, GHC.Generics.to is just a coercion.