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

[Richard Eisenberg]

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