Unused type variables with non-linear patterns

Simon Peyton Jones simonpj at microsoft.com
Tue Dec 15 16:02:28 UTC 2015

I agree too.

Is there a ticket?   Does it emit a warning right now?


From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Richard Eisenberg
Sent: 11 December 2015 16:02
To: Michael Sloan <mgsloan at gmail.com>
Cc: ghc-devs at 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.


On Dec 10, 2015, at 4:32 PM, Michael Sloan <mgsloan at gmail.com<mailto: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-<https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fhackage.haskell.org%2fpackage%2fbase->


On Thu, Dec 10, 2015 at 6:48 AM, Jan Stolarek <jan.stolarek at p.lodz.pl<mailto: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?


Politechnika Łódzka
Lodz University of Technology

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