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

[Simon Peyton Jones]

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<mailto: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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