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

[Simon Marlow]

I use MultiWayIf occasionally, but I would probably not use it at all if I
had to type "\mcase" rather than "if".  I mean, that just feels too obscure
and ugly - yes it's kind of cute that it falls out as the degenerate case
of zero arguments, but I find it strange that I could write something that
looks like a lambda and not get a lambda, and "mcase" is just
obscure-sounding.

I probably wouldn't object all that much to getting rid of MultiWayIf, it
might not be paying its way, but I'm not convinced that \mcase or \of would
pay their way either.

Cheers
Simon


On Thu, 17 Sep 2020 at 16:22, Simon Peyton Jones <simonpj at microsoft.com>
wrote:

> If it was re-cast as \mcase, which is just like \case but allows n-ary
> functions, I’d find it quite acceptable.  The two then become extremely
> close, so there’s a very low cognitive load.
>
>
>
> GHC’s internals already allow this, and it seems surprisingly
> non-orthogonal that the source language does not.
>
>
>
> We could kill off MultiWayIf.
>
>
>
> But I don’t feel strongly.  If a consensus does not emerge, maybe we
> should just vote.
>
>
>
> Simon
>
>
>
> *From:* ghc-steering-committee <ghc-steering-committee-bounces at haskell.org>
> *On Behalf Of *Simon Marlow
> *Sent:* 17 September 2020 15:53
> *To:* Richard Eisenberg <rae at richarde.dev>
> *Cc:* Simon Peyton Jones via ghc-steering-committee <
> ghc-steering-committee at haskell.org>
> *Subject:* Re: [ghc-steering-committee] Proposal #302: `\of`
>
>
>
> Cale's rationale chimes with me. A lot - I feel like I might have even
> made the same point in previous threads on this. I think of the tradeoff
> like this:
>
>
>
> * The lack of \of doesn't really hurt very much. In fact, arguably by
> forcing the author to type some more characters and give something a name,
> we get code that's clearer for the reader. (yes this is very subjective,
> but syntax is).
>
> * The addition of \of *would* hurt new users of the language. Only a bit,
> but every bit makes things worse, and things are already quite bad.
>
>
>
> Cheers
>
> Simon
>
>
>
> On Thu, 3 Sep 2020 at 23:02, Richard Eisenberg <rae at richarde.dev> wrote:
>
> 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
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F302&data=02%7C01%7Csimonpj%40microsoft.com%7C956b41eaa3984a4a739d08d85b197bb4%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637359513199745852&sdata=dTBwWAv4oSLAVZXcCimFWfeNygnz%2FKcadcWw0YNdcuk%3D&reserved=0>
> Proposal:
> https://github.com/JakobBruenker/ghc-proposals/blob/patch-1/proposals/0000-lambda-layout.md
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FJakobBruenker%2Fghc-proposals%2Fblob%2Fpatch-1%2Fproposals%2F0000-lambda-layout.md&data=02%7C01%7Csimonpj%40microsoft.com%7C956b41eaa3984a4a739d08d85b197bb4%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637359513199755846&sdata=MusKqbiaRHp2rbPuyFQTYYiSP9%2FjGkwKKmPoPwRu1qc%3D&reserved=0>
> Cale's recommendation:
> https://github.com/ghc-proposals/ghc-proposals/pull/302#issuecomment-666075014
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F302%23issuecomment-666075014&data=02%7C01%7Csimonpj%40microsoft.com%7C956b41eaa3984a4a739d08d85b197bb4%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637359513199755846&sdata=MiDN5T22uVYITKYiB3yyyqrhe%2FleEKLvJk%2FjnaiVI7Y%3D&reserved=0>
>
> 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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