[ghc-steering-committee] Proposal #302: `\of`

[Alejandro Serrano Mena]

I think that would work pretty well. In fact, the implementation of such a
rule would allow us to also point beginners when they need parentheses in
regular pattern matching!

El vie., 4 sept. 2020 a las 16:55, Richard Eisenberg (<rae at richarde.dev>)
escribió:

> Thinking about disambiguating \case Just x -> x: There is no way that \of
> Just x -> ... can be correct, because Just requires an argument. Very
> happily, the renamer's behavior is unaffected: any variables introduced are
> still binding sites. So, I think we could probably detect a case like this
> in the type-checker and either reinterpret it correct or provide a very
> helpful error message.
>
> So here's a concrete idea:
>
> - Change the existing proposal to use \case instead of \of.
> - Add a little magic to the type-checker, as specified below, to allow
> current usages of \case to be accepted.
> - When the magic triggers, issue a warning (-Wdeprecated-lambda-case, on
> by default).
> - Remove the magic (and its warning) in a few releases.
>
> The magic:
> - The new construct allows many patterns before the -> where the old \case
> allowed only one.
> - The type-checker detects when the first of these patterns is a solitary
> constructor, and when this constructor is not nullary.
> - It then reconstructs the AST to move the remaining patterns as arguments
> of that first one. Because our AST tracks parens manually, the new AST will
> even print identically to the old one.
>
> This magic is, well, magical, but it's well specified, backward
> compatible, and unambiguous.
>
> What do we think?
>
> Richard
>
> On Sep 4, 2020, at 10:37 AM, Alejandro Serrano Mena <trupill at gmail.com>
> wrote:
>
> Hi all,
> I agree with the sentiment. I think adding \of would be a nice way to
> clean up the language (in the same way that now StandaloneKindSignatures
> allow us to remove the weirder CUSKs), but unless we have some deprecation
> policy for extensions, this may confuse people more than help.
>
> It's particularly bad that we cannot make this simply an extension of
> \case, due to the example pointed out in the thread: \case Just x -> x.
> Could we maybe think of some way to disambiguate those cases? I'm going to
> ask this in the thread too.
>
> Regards,
> Alejandro
>
> El vie., 4 sept. 2020 a las 0:02, Richard Eisenberg (<rae at richarde.dev>)
> escribió:
>
>> Hi all,
>>
>> Proposal #302 was submitted to the committee and assigned to Cale. He has
>> made a recommendation on the GitHub trail, but I don't believe the
>> committee has discussed this among ourselves.
>>
>> PR: https://github.com/ghc-proposals/ghc-proposals/pull/302
>> Proposal:
>> https://github.com/JakobBruenker/ghc-proposals/blob/patch-1/proposals/0000-lambda-layout.md
>> Cale's recommendation:
>> https://github.com/ghc-proposals/ghc-proposals/pull/302#issuecomment-666075014
>>
>> The idea, in brief, is to introduce a new syntax (guarded behind
>> -XMultiWayLambda) \of. Here is an example, which gives you the idea:
>>
>> mplus :: Maybe Int -> Maybe Int -> Maybe Int
>> mplus = \of
>>   Nothing _ -> Nothing
>>   _ Nothing -> Nothing
>>   (Just x) (Just y) -> Just (x + y)
>>
>> The new keyword allows us to use a syntax similar to function
>> definitions, but without repeating the name of the function. It is also
>> like \case, but it allows multiple arguments. Guards are allowed, as usual.
>>
>> I really like this new syntax -- mostly because I find it very strange
>> that we have to repeat the function name on every line. And then change the
>> spacing when we change the function name. And I like the mnemonic "lambda
>> of". And it allows me to write a where clause that is accessible in
>> multiple different patterns, or an indented where clause that is usable in
>> just one. If it didn't confuse readers, I would use this syntax all the
>> time.
>>
>> Even so, I agree with Cale's recommendation to reject. We just have too
>> much syntax! If someone were to come along and draft a concrete proposal of
>> how we could, for example, use this syntax to replace both \case and if|,
>> with a migration strategy, etc., then I might be in favor. Until then, I
>> think we've spent our budget for cute, obscure bits of syntax.
>>
>> Richard
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>>
>
>
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