Re: Unused type variables with non-linear patterns

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

--- Politechnika Łódzka Lodz University of Technology

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