I agree with Michael. This should not emit a warning. While you're in this area, make sure that

type instance F _ _ = Int

indeed matches `F Char Bool`. It's easy to imagine both underscores being treated as the same variable, which is not what the user intends. Thanks, Richard On Dec 10, 2015, at 4:32 PM, Michael Sloan wrote:

I definitely think this should be considered a usage. Ideally, you should always be able to resolve warnings, but there'd be no straightforward way to do so if LHS-only usages cause warnings. Also, It's quite useful to use type variables only on the LHS. Usage example: https://hackage.haskell.org/package/base-4.8.1.0/docs/src/Data.Type.Equality...

-Michael

On Thu, Dec 10, 2015 at 6:48 AM, Jan Stolarek wrote: An intersting thing came up when implementing warnings for unused type variables in type family patterns (#10982 / D1576):

type family X a b where X a a = Int

Should we emit a warning that `a` is unused? It does not appear in the RHS but it is actually used because of non-linear pattern. We can fix this by prefixing both appearances of `a` with an underscore - this silences the warning and maintains semantics. Nevertheless, if a variable is unused it should be perfectly valid to replace it with an underscore - that is not the case in this example. Thoughts?

Janek

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Is there a ticket? #10982

Janek --- Politechnika Łódzka Lodz University of Technology Treść tej wiadomości zawiera informacje przeznaczone tylko dla adresata. Jeżeli nie jesteście Państwo jej adresatem, bądź otrzymaliście ją przez pomyłkę prosimy o powiadomienie o tym nadawcy oraz trwałe jej usunięcie. This email contains information intended solely for the use of the individual to whom it is addressed. If you are not the intended recipient or if you have received this message in error, please notify the sender and delete it from your system.