[Haskell-cafe] Let's teach GHC idiom brackets!

Elliot Cameron elliot.cameron at covenanteyes.com
Wed Feb 18 20:20:32 UTC 2015

As a beginner Haskeller, I think my perspective might be surprising: I actually think idiom brackets would make learning and understanding Haskell *easier* for new-comers. Applicative code is intimidating to read for beginners and I have often been afraid to use it because it all seems so magical (lots of strange operators between everything!). But when I read the papers and saw idiom brackets, I suddenly realized that the whole idea was actually pretty simple! I've been pushing for them ever since. 

----- Original Message -----

From: "Oliver Charles" <ollie at ocharles.org.uk> 
To: "Haskell Cafe" <haskell-cafe at haskell.org> 
Sent: Wednesday, February 18, 2015 2:46:16 PM 
Subject: [Haskell-cafe] Let's teach GHC idiom brackets! 

Hi friends, 

Last night I was having a good natter with Chris Done, and the topic of idiom brackets came up. Specifically, Chris' hindent and structured-haskell-mode tools have both recently become aware of the quasiquoter in applicative-quoters - allowing one to write ordinary Haskell code but have it formatted/edited as if it wasn't even inside a quasiquoter. However, it's a shame that Chris has to now be aware of this specific quasiquoter just to get his tools to work properly. Furthermore, it's a shame that everyone has to do the same whenever they want to manipulate Haskell code. 

That said, idiom brackets - at least to me - appears to be a very desirable syntax extension. A recent desire for this came to me while working with reactive-banana. As almost everything "interesting" is even an Event or a Behavior, there tends to be a lot of assembling programs with applicative syntax. Another obvious example is when using parser combinators. 

Therefore, I'd like to propose that we teach GHC about this desugaring, properly. 

Now, how we do that, and in fact, what I'm even proposing... that's all up for discussion! I spoke briefly with David Christiansen and a few others in the Idris IRC channel last night about their particular variation of idiom brackets, and it seems people are generally happy with that. Copying that almost verbatim is probably the first proposal I'd put forward. 

This is the first time I've proposed an extension to GHC, so I'll need some guidance to make this happen - both in terms of guiding the discussion through to a proposal, but ultimately implementing it. Perhaps the proposal will be ready for ZuriHac, and some GHC devs could help me work on it there. 

Anyway, that's enough rambling from me. I would love to hear your thoughts - do you think idiom brackets are a good idea? Do you have a particular tricky case that you think they need to support? I'm all ears. 

-- Ollie 

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