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

Bas van Dijk v.dijk.bas at gmail.com
Wed Feb 18 21:41:50 UTC 2015

Do we still need idiom brackets if we have ApplicativeDo?




On 18 February 2015 at 20:46, Oliver Charles <ollie at ocharles.org.uk> wrote:
> 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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