suppress warning "Defined but not used: type variable ‘x’" in GHC-8.0 - Glasgow-haskell-users - Haskell.org

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

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Henning Thielemann

9 Jan 2016 9 Jan '16

5:57 p.m.

GHC-8.0 emits several new warnings of this kind: Defined but not used: type variable ‘x’ for declarations like type instance Snd x y = y Enthusiastically, I started to replace unused type function arguments by underscores, only to find out that older GHC versions do not accept that. With what option can I disable this warning? Or can it be removed from -Wall for now?

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Carter Schonwald

9 Jan 9 Jan

6:06 p.m.

Have you tried _x instead? On Saturday, January 9, 2016, Henning Thielemann < lemming@henning-thielemann.de> wrote:

GHC-8.0 emits several new warnings of this kind:

Defined but not used: type variable ‘x’

for declarations like

type instance Snd x y = y

Enthusiastically, I started to replace unused type function arguments by underscores, only to find out that older GHC versions do not accept that. With what option can I disable this warning? Or can it be removed from -Wall for now?

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Henning Thielemann

6:44 p.m.

On Sat, 9 Jan 2016, Carter Schonwald wrote:

Have you tried _x instead?

Ah, this solves the problem! Almost. I have an instance like this one: 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.

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Richard Eisenberg

11 Jan 11 Jan

12:42 p.m.

On Jan 9, 2016, at 6:44 PM, Henning Thielemann 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. Richard

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Henning Thielemann

2:12 p.m.

On Mon, 11 Jan 2016, Richard Eisenberg wrote:

On Jan 9, 2016, at 6:44 PM, Henning Thielemann 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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Edward Kmett

17 Jan 17 Jan

12:24 a.m.

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@henning-thielemann.de> wrote:

On Mon, 11 Jan 2016, Richard Eisenberg wrote:

On Jan 9, 2016, at 6:44 PM, Henning Thielemann <

...
lemming@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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Andrew Farmer

3:16 a.m.

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" 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@henning-thielemann.de> wrote:

...
On Mon, 11 Jan 2016, Richard Eisenberg wrote:

On Jan 9, 2016, at 6:44 PM, Henning Thielemann <

...
lemming@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.

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users

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Edward Kmett

9:44 a.m.

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 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" 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@henning-thielemann.de> wrote:

...
On Mon, 11 Jan 2016, Richard Eisenberg wrote:

On Jan 9, 2016, at 6:44 PM, Henning Thielemann <

...
lemming@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.

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users

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Edward Kmett

9:45 a.m.

Moreover those _'d type variables would infect all of our haddocks.

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Oliver Charles

5:20 p.m.

Inferred types and errors too, I would imagine. On Sun, 17 Jan 2016 2:45 pm Edward Kmett wrote:

Moreover those _'d type variables would infect all of our haddocks. _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users

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Simon Peyton Jones

18 Jan 18 Jan

4:57 a.m.

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@haskell.org] On Behalf Of Edward Kmett Sent: 17 January 2016 14:44 To: Andrew Farmer Cc: Henning Thielemann ; glasgow-haskell-users@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 mailto:xichekolas@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" mailto:ekmett@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 mailto:lemming@henning-thielemann.de> wrote: On Mon, 11 Jan 2016, Richard Eisenberg wrote: On Jan 9, 2016, at 6:44 PM, Henning Thielemann mailto:lemming@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. _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.orgmailto:Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.haskell.org%2fcgi-bin%2fmailman%2flistinfo%2fglasgow-haskell-users&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c560da21ad3be4ab474ca08d31f4cab95%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=SUPkAvfyDQt84m9WwO%2b0z%2fDVA5nWDp7u28%2bkOXt0d94%3d _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.orgmailto:Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.haskell.org%2fcgi-bin%2fmailman%2flistinfo%2fglasgow-haskell-users&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c560da21ad3be4ab474ca08d31f4cab95%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=SUPkAvfyDQt84m9WwO%2b0z%2fDVA5nWDp7u28%2bkOXt0d94%3d

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participants (7)

Andrew Farmer
Carter Schonwald
Edward Kmett
Henning Thielemann
Oliver Charles
Richard Eisenberg
Simon Peyton Jones