[ghc-steering-committee] Language Extension Policy – Round 2

[Adam Gundry]

On 27/04/2023 15:11, Eric Seidel wrote:
> 
> I can't speak for Simon, but I felt similarly about Extensions vs
> Warnings. What you're saying here is not quite what I had in mind.
> For me it's more like
> 
>> • Remove (say) MultiParamTypeClasses as an extension
>> • Accept programs that use MultiParamTypeClasses unconditionally
>> • If you want to disallow MPTCs, you can disable them with
>> `-Wmulti-param-type-classes -Werror`
> 
> In other words, MPTCs are simply part of the language now. Not a
> second class citizen that must be opted into with an Extension or by
> *disabling* a Warning. We leave an escape hatch in the form of an
> opt-in warning for those who would rather not use MPTCs, much like
> other aspects of the language the may be controversial.

Here's what I don't quite understand: MultiParamTypeClasses is already 
part of GHC2021 and thus enabled by default (unless the program is 
specifying e.g. Haskell2010 as the default-language). Given that, why is 
it worthwhile to go through the churn of replacing 
`-XNoMultiParamTypeClasses` with `-Werror=multi-param-type-classes`?

If the issue is implementation complexity, then I don't see how changing 
one flag to another really helps that; to fundamentally simplify the 
implementation you have to eliminate the ability to disable the feature 
altogether. (And I would argue that the way to keep the implementation 
simple is to design language features that are orthogonal, rather than 
having one big monolithic language that can't be decomposed...)

If it's a conceptual/documentation issue, could we instead rewrite the 
user's guide so that it assumes GHC2021 as a baseline, and presents 
multi-parameter type classes as a feature of "GHC Haskell" (with a side 
note that they can be disabled with NoMultiParamTypeClasses)?

I'm sure I'm missing something... :-)

Adam

-- 
Adam Gundry, Haskell Consultant
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