I agree too.
Is there a ticket? Does it emit a warning right now?
Simon
From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Richard Eisenberg
Sent: 11 December 2015 16:02
To: Michael Sloan
Cc: ghc-devs@haskell.org
Subject: 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 mailto:mgsloan@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...https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fhackage.haskell.org%2fpackage%2fbase-4.8.1.0%2fdocs%2fsrc%2fData.Type.Equality.html%23EqStar&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c65bff682e66a434f1f1708d302447d3f%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=gkgghrbPZ7shUtk4z2WesVVJaQD8wlKhlbXWmFHpjEs%3d
-Michael
On Thu, Dec 10, 2015 at 6:48 AM, Jan Stolarek mailto:jan.stolarek@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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