No hidden Bool here -- this is just a consequence of the way that view patterns work, where you have to match against the result of the function, in this case, (>0). See https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/view_pa... Richard

On Mar 12, 2021, at 6:37 AM, Anthony Clayden wrote:

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

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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.)

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