How is default method signatures supposed to be used together with Generics

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Johan Tibell

11 Dec 2012 11 Dec '12

11:28 p.m.

Hi, I noticed that you're not required to export the types mentioned in the default method signature. For example, you could have: default hashWithSalt :: (Generic a, GHashable (Rep a)) => Int -> a -> Int hashWithSalt salt = ghashWithSalt salt . from and not export the GHashable class. However, if users try to define an instance of Hashable but forget to derive Generic: data Foo a = Foo a String deriving (Eq) -- Oops, forgot Generic here instance (Hashable a) => Hashable (Foo a) they get a pretty bad error message: Test.hs:10:10: Could not deduce (Generic (Foo a), Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a))) arising from a use of `Data.Hashable.Class.$gdmhashWithSalt' from the context (Hashable a) bound by the instance declaration at Test.hs:10:10-41 Possible fix: add (Generic (Foo a), Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a))) to the context of the instance declaration or add instance declarations for (Generic (Foo a), Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a))) In the expression: (Data.Hashable.Class.$gdmhashWithSalt) In an equation for `hashWithSalt': hashWithSalt = (Data.Hashable.Class.$gdmhashWithSalt) In the instance declaration for `Hashable (Foo a)' Exporting GHashable would help a little bit in that users would at least know what this GHashable class that the error talks about is. What's best practice? -- Johan

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dag.odenhall＠gmail.com

12 Dec 12 Dec

12:31 a.m.

New subject: How is default method signatures supposed to be used together with Generics

The practice seems to be to not export it, but maybe it would be a better practice to export it. That way it can work without DefaultSignatures too, and if you use the generic-deriving package it could work with zero extensions or GHC-specific dependencies. Are you adding support to hashable after all? I'd love that, but thought you decided against it because of the clash with the existing defaults. On Wed, Dec 12, 2012 at 5:28 AM, Johan Tibell wrote:

...

Hi,

I noticed that you're not required to export the types mentioned in the default method signature. For example, you could have:

default hashWithSalt :: (Generic a, GHashable (Rep a)) => Int -> a -> Int hashWithSalt salt = ghashWithSalt salt . from

and not export the GHashable class. However, if users try to define an instance of Hashable but forget to derive Generic:

data Foo a = Foo a String deriving (Eq) -- Oops, forgot Generic here

instance (Hashable a) => Hashable (Foo a)

they get a pretty bad error message:

Test.hs:10:10: Could not deduce (Generic (Foo a), Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a))) arising from a use of `Data.Hashable.Class.$gdmhashWithSalt' from the context (Hashable a) bound by the instance declaration at Test.hs:10:10-41 Possible fix: add (Generic (Foo a), Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a))) to the context of the instance declaration or add instance declarations for (Generic (Foo a), Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a))) In the expression: (Data.Hashable.Class.$gdmhashWithSalt) In an equation for `hashWithSalt': hashWithSalt = (Data.Hashable.Class.$gdmhashWithSalt) In the instance declaration for `Hashable (Foo a)'

Exporting GHashable would help a little bit in that users would at least know what this GHashable class that the error talks about is.

What's best practice?

-- Johan

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

dag.odenhall＠gmail.com

12:32 a.m.

New subject: How is default method signatures supposed to be used together with Generics

To be clear, I think DefaultSignatures should still be an option, but possibly guarded by some CPP. On Wed, Dec 12, 2012 at 6:31 AM, dag.odenhall@gmail.com < dag.odenhall@gmail.com> wrote:

...

The practice seems to be to not export it, but maybe it would be a better practice to export it. That way it can work without DefaultSignatures too, and if you use the generic-deriving package it could work with zero extensions or GHC-specific dependencies.

Are you adding support to hashable after all? I'd love that, but thought you decided against it because of the clash with the existing defaults.

On Wed, Dec 12, 2012 at 5:28 AM, Johan Tibell wrote:

...
Hi,

I noticed that you're not required to export the types mentioned in the default method signature. For example, you could have:

default hashWithSalt :: (Generic a, GHashable (Rep a)) => Int -> a -> Int hashWithSalt salt = ghashWithSalt salt . from

and not export the GHashable class. However, if users try to define an instance of Hashable but forget to derive Generic:

data Foo a = Foo a String deriving (Eq) -- Oops, forgot Generic here

instance (Hashable a) => Hashable (Foo a)

they get a pretty bad error message:

Test.hs:10:10: Could not deduce (Generic (Foo a), Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a))) arising from a use of `Data.Hashable.Class.$gdmhashWithSalt' from the context (Hashable a) bound by the instance declaration at Test.hs:10:10-41 Possible fix: add (Generic (Foo a), Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a))) to the context of the instance declaration or add instance declarations for (Generic (Foo a), Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a))) In the expression: (Data.Hashable.Class.$gdmhashWithSalt) In an equation for `hashWithSalt': hashWithSalt = (Data.Hashable.Class.$gdmhashWithSalt) In the instance declaration for `Hashable (Foo a)'

Exporting GHashable would help a little bit in that users would at least know what this GHashable class that the error talks about is.

What's best practice?

-- Johan

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Johan Tibell

12:34 p.m.

New subject: How is default method signatures supposed to be used together with Generics

On Tue, Dec 11, 2012 at 9:31 PM, dag.odenhall@gmail.com wrote:

...

The practice seems to be to not export it, but maybe it would be a better practice to export it. That way it can work without DefaultSignatures too, and if you use the generic-deriving package it could work with zero extensions or GHC-specific dependencies.

Are you adding support to hashable after all? I'd love that, but thought you decided against it because of the clash with the existing defaults.

We did a major redesign of the library (without affecting users much). This meant that hash became a top-level function and hashWithSalt the only method of the Hashable class. As it's now the only method we don't have a default implementation for it anymore (except the default method signature). -- Johan

José Pedro Magalhães

4:23 a.m.

New subject: How is default method signatures supposed to be used together with Generics

Hi Johan, The error message is not ideal, but it does say that adding a Generic (Foo a) instance might solve the problem. I generally do not export classes like GHashable because they are closed; users should never need to provide more instances themselves. Also, exporting the class won't make the error message any better, and the solution to this error doesn't really involve the GHashable class... Cheers, Pedro On Wed, Dec 12, 2012 at 4:28 AM, Johan Tibell wrote:

...

Hi,

I noticed that you're not required to export the types mentioned in the default method signature. For example, you could have:

default hashWithSalt :: (Generic a, GHashable (Rep a)) => Int -> a -> Int hashWithSalt salt = ghashWithSalt salt . from

and not export the GHashable class. However, if users try to define an instance of Hashable but forget to derive Generic:

data Foo a = Foo a String deriving (Eq) -- Oops, forgot Generic here

instance (Hashable a) => Hashable (Foo a)

they get a pretty bad error message:

Test.hs:10:10: Could not deduce (Generic (Foo a), Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a))) arising from a use of `Data.Hashable.Class.$gdmhashWithSalt' from the context (Hashable a) bound by the instance declaration at Test.hs:10:10-41 Possible fix: add (Generic (Foo a), Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a))) to the context of the instance declaration or add instance declarations for (Generic (Foo a), Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a))) In the expression: (Data.Hashable.Class.$gdmhashWithSalt) In an equation for `hashWithSalt': hashWithSalt = (Data.Hashable.Class.$gdmhashWithSalt) In the instance declaration for `Hashable (Foo a)'

Exporting GHashable would help a little bit in that users would at least know what this GHashable class that the error talks about is.

What's best practice?

-- Johan

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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dag.odenhall＠gmail.com
Johan Tibell
José Pedro Magalhães