Unused type variables with non-linear patterns

[Richard Eisenberg]

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 <mgsloan at gmail.com> 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.html#EqStar
> 
> -Michael
> 
> On Thu, Dec 10, 2015 at 6:48 AM, Jan Stolarek <jan.stolarek at p.lodz.pl> 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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