-XDeriveTraversable should imply -XDeriveFunctor and -XDeriveFoldable

In the discussion in #2953 (closed) it was concluded that it would be best to have separate flags for deriving Functor, Foldable and Traversable. I agree with that conclusion, but it seems that it wasn't considered to have -XDeriveTraversable imply -XDeriveFunctor and -XDeriveFoldable. I can't think of a reason not to want that.

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Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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changed weight to 5

added Tfeature request Trac import labels

I.e. add this to the impliedFlags list:

    , (Opt_DeriveTraversable, turnOn, Opt_DeriveFunctor)
    , (Opt_DeriveTraversable, turnOn, Opt_DeriveFoldable)

Attached file patch-9069.diff (download).

Note: this is my first GHC patch!

Ah -- but you forgot to update the user manual!

Also can you just articulate the reasoning behind having -XDeriveTraversable imply the other two, and perhaps summarise that reasoning in the user-manual entry?

Thanks

Simon

Right, thanks, I will do that!

The main reason is that currently you have to quite a bit to just get a derived Traversable instance:

1. You have to add deriving (Functor, Foldable, Traversable)
2. You have to import Data.Foldable and Data.Traversable
3. You have to turn on 3 extensions.

Item 1 is fine, we don't want any magic. Item 2 could be solved by moving Foldable and Traversable to the prelude. So the only thing that I can do right now is improve item 3.

If you derive the Traversable instance it would be invalid to have an instance for Functor or Foldable that is not equiavalent to the derived instances, so it makes sense to turn those extensions on automatically.

Attached file patch-9069.2.diff (download).

That second patch replaces the first. (I forgot to check the checkmark, and there seems to be no way to fix it afterwards.)

By the way, how can I test the documentation? Edit: Never mind, found the appropriate page on the wiki...

OK, thanks.

You missed the flag reference summary list Section 4.20.12, where flag implications should be summarised. But that's not surprising since DeriveTraversable and friends aren't even listed there.

Could you do us a favour?

• Add the DeriveX extensions to that table
• Put the whole table in alphabetical order of extension name
• Check that there are no more missing one
• Check that all implications are listed.

That would be a very useful service. It could be a separate patch.

Meanwhile, Austin, please commit.

Simon

Attached file language-options-docs-patch.diff (download).

Updates to the documentation

It might be a good idea to not to wait too long to merge the documentation patch in. Because the whole language options table is reordered any change to it will create a merge conflict.

Would someone at ZuriHac like to do this?

Will have a look.

mentioned in commit 3bdc78b5

mentioned in commit 63d7047a

closed

Looked fined and validated; pushed.

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Trac field	Value
Resolution	Unresolved → ResolvedFixed

changed milestone to %7.10.1

added Pnormal label