Accumulate errors in <!>

[gwils]

sv uses Validation to accumulate multiple errors in Applicative operations.
For example, consider a <*> b <*> c. If more than one of these fails, the errors from all of them are collected.
Now consider the Alt operation x <!> y <!> z. In this case, if more than one fails, only the errors from the last (z) are collected.

It would be better if all the errors were kept. But I speculate that having all the <*> and <!> errors show up at the same level could quite confusing and unclear. I wonder if we could build a tree structure full of errors, or otherwise tag that a group of errors came from a chain of <!>

[phadej]

https://github.com/futurice/haskell-mega-repo/blob/f660b3dfc136d5e3b85f40766297a2897955f4ec/env-config/src/Futurice/EnvConfig.hs#L282-L308

And then you'll need a Lattice. I use a CNF, but a a tree would do too (which would preserve more of error structure).