Change prerender return to expose initial value

[alexfmpe]

As is, this is a breaking change. Migration strategy is pretty much to add a uncurry holdDyn in front of prerender.

A non-breaking alternative is to add a separate function, say, prerender', though that makes more work for the implementation:

• we can supply a default method implementation prerender a b = uncurry holdDyn =<< prerender' a b, but that would add the MonadHold constraint to all the implementations that don't supply their own prerender, which might be its own breaking change. For instance, these two instances would now require extra constraints on the function that tries to use them
  • instance (SupportsStaticDomBuilder t m) => Prerender t (StaticDomBuilderT t m) where
  • instance (Prerender t m, Monad m) => Prerender t (ReaderT r m) where
• we can also factor out the common code to avoid extra constraints

  instance (ReflexHost t, Adjustable t m, PrerenderBaseConstraints t m) => Prerender t (HydrationDomBuilderT GhcjsDomSpace t m) where
    type Client (HydrationDomBuilderT GhcjsDomSpace t m) = HydrationDomBuilderT GhcjsDomSpace t m
    prerender a b = pure <$> commonImpl a b
    prerender' a b = (commonImpl a b, never)
  
  commonImpl :: m a -> Client m a -> m a
  commonImpl _ client = client

but the lack of let bindings in classes makes this very awkward.

I'm not sure the pain to make this non-breaking is worth it especially given we'd then end with 3 very similar functions:

• prerender
• prerender_
• prerender'