Thank you Richard, Lennart,
Gergő
> pattern Positive :: (Ord a, Num a) => a
> pattern Positive <- ((>0) -> True)
Heh heh, there's another surprise/undocumented 'feature'.
It's not necessary to give a signature for pattern `Positive`, GHC will infer that from the decl.
I was surprised to see `True`, and even more surprised there wasn't a `Bool` in the signature. I guess that's so `Positive` can appear as a pattern in a case expr. To dig out the positive value in a lambda expr, it seems I go
> (\p@Positive -> p) 5 -- returns 5
Seems I can't use any trick like that to turn `Positive` into explicitly bidirectional. I also tried
> pattern Positive' <- ((>0) -> ())
But that's rejected '"* Couldn't match expected type `Bool' with actual type `()'".
Is the hidden `Bool` documented somewhere? (Doesn't seem to be in the User Guide nor the wiki nor the paper, on a quick scan.)