RE: suppress warning "Defined but not used: type variable ‘x’" in GHC-8.0

[Simon Peyton Jones]

I have created not one but three tickets arising from this thread:


·         https://ghc.haskell.org/trac/ghc/ticket/11449

·         https://ghc.haskell.org/trac/ghc/ticket/11450

·         https://ghc.haskell.org/trac/ghc/ticket/11451

I’d love comments on them: which of the three matter most to you folk?  The first also involves a flag-naming question.

Simon

From: Glasgow-haskell-users [mailto:glasgow-haskell-users-bounces at haskell.org] On Behalf Of Edward Kmett
Sent: 17 January 2016 14:44
To: Andrew Farmer <xichekolas at gmail.com>
Cc: Henning Thielemann <lemming at henning-thielemann.de>; glasgow-haskell-users at haskell.org
Subject: Re: suppress warning "Defined but not used: type variable ‘x’" in GHC-8.0

No, the type instance must match the class heading.

I can use

instance Foo [_a] where
  type Bar [_a] = Int
  whatever = ... where
    bar :: _a -> Int
    bar = ...

but that is a needlessly messy thing to request of every package everywhere.

The arguments being pattern matched in a class associated type aren't really just bindings, they reference the surrounding context and so shouldn't be treated the same as the basic type family case.

It isn't just the class associated type being mangled, it is every type variable that comes from the instance head in the entire body of every instance that happens to have a class associated type in it.

Note that in the example above I added another ScopedTypeVariables reference to the same parameter, but it _also_ must be mangled despite having absolutely nothing to do with the class associated type.

The existing convention has worked since 6.10 or so, when all of this stuff was invented in the first place, and the new state of affairs is clearly worse.

-Edward

On Sun, Jan 17, 2016 at 3:16 AM, Andrew Farmer <xichekolas at gmail.com<mailto:xichekolas at gmail.com>> wrote:

Can't you just:

instance Foo [a] where
  type Bar [_a] = Int

(At least I think I did that somewhere...)
On Jan 16, 2016 9:24 PM, "Edward Kmett" <ekmett at gmail.com<mailto:ekmett at gmail.com>> wrote:
As a data point I now get thousands of occurrences of this warning across my packages.

It is quite annoying.

class Foo a where
  type Bar a

instance Foo [a] where
  type Bar [a] = Int

is enough to trigger it.

And you can't turn it off by using _ as

instance Foo [_] where
  type Bar [_] = Int

isn't legal.

I've been avoiding it for now by using

  if impl(ghc >= 8)

    ghc-options: -fno-warn-unused-matches

but this is a pretty awful addition to this warning as it stands.
-Edward

On Mon, Jan 11, 2016 at 2:12 PM, Henning Thielemann <lemming at henning-thielemann.de<mailto:lemming at henning-thielemann.de>> wrote:

On Mon, 11 Jan 2016, Richard Eisenberg wrote:
On Jan 9, 2016, at 6:44 PM, Henning Thielemann <lemming at henning-thielemann.de<mailto:lemming at henning-thielemann.de>> wrote:

instance (Natural n) => Num.Integer (Un n) where
   type Repr (Un _n) = Unary


GHC-7.6.3 and GHC-7.4.2 complain:
   Type indexes must match class instance head
   Found `Un _n' but expected `Un n'
   In the type synonym instance declaration for `Num.Repr'
   In the instance declaration for `Num.Integer (Un n)'


GHC-7.8.4, GHC-7.10.3 and GHC-8.0 are happy with the difference.

I'm surprised this is accepted at all. Looks like hogwash to me. I think you should post a bug report.

Ok, but then GHC must not warn about the unused argument of Repr.

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