Auto-derive SeqIsEval for Generic type

[aspiwack]

Idea suggested by @Profpatsch on twitter

We could add an instance instance (Generic a, …) => SeqIsEval a. This instance would check that every field is strict (is it possible in the generic framework? I expect it is. Though I imagine that it's a little bit of work).

This instance is overlapping, but there is no risk in overlapping instances here: SeqIsEval has trivial evidence, so is automatically coherent. On the other hand, we need to be a little careful with error messages, because the instance will always trigger. It needs to point the user in the right direction if it eventually fails.

[TeofilC]

There's a similar idea in this issue haskell/deepseq#3 .
I developed the gist from there a bit to handle recursion here: https://gist.github.com/TeofilC/a85da6a388ec94e8acb1519886fbd2a7

A slightly different approach is taken here: https://gist.github.com/anka-213/440cfef8c8712f5df747029640cfe170

One thing to also watch out for is that newtypes need to be handled as a special case as their field is always DecidedLazy (cf. https://gitlab.haskell.org/ghc/ghc/-/issues/21380)

Hope these links help!

[aspiwack]

Beautiful! This is all very helpful, thank you. Especially the newtype thing. Newtypes are easy to detect in Generic if I remember correctly, but it's would be quite easy to overlook.

[TeofilC]

I've also recently published a library that uses TH to do this stuff that you might find useful: https://tracsis.github.io/th-deepstrict/

It's not on Hackage yet but should be in a couple of months