Unused type variables with non-linear patterns

[Michael Sloan]

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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