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2 points by rocketnia 546 days ago | link | parent

"Isn't there a dialect out there that uses a different bracket to mean 'close all open parens'? So that the example above would become (foo (a b c (d e f]. I can't quite place the memory."

I was thinking about that and wanted to find a link earlier, but I can't find it. I hope someone will; I want to add a credit to that idea in Parendown's readme.

I seem to remember it being one of the early ideas for Arc that didn't make it to release, but maybe that's not right.


"I do like far better the idea of replacing withs with a '/' ssyntax for let."

I like it pretty well!

It reminds me of a `lets` macro I made in Lathe for Arc:

    a (bind 1)
    (expr 1)
    b (bind 2)
    (expr 2)
    c (bind 3)
    (expr 3))
I kept trying to find a place this would come in handy, but the indentation always felt weird until I finally prefixed the non-binding lines as well with Parendown.

This right here

    (expr 1)
    b (bind 2)
looked like the second ( was nested inside the first, so I think at the time I tried to write it as

       (expr 1)
    b  (bind 2)
With my implementation of Penknife for JavaScript, I finally started putting /let, /if, etc. at the beginning of every line in the "block" and the indentation was easy.

With the syntax you're showing there, you at least have a punctuation character at the start of a line, which I think successfully breaks the illusion of nested parens for me.


It looks like I implemented `lets` because of this thread:

And would you look at that, ylando's macro (scope foo @ a b c @ d e f) is a shallow version of Parendown's macro (pd @ foo @ a b c @ d e f). XD (The `pd` macro works with any symbol, and I would usually use `/`.) I'm gonna have to add a credit to that in Parendown's readme too.

Less than a year later than that ylando thread, I started a Racket library with a macro (: foo : a b c : d e f):

So I suppose I have ylando to thank for illustrating the idea, as well as fallintothis for interpreting ylando's idea as an implemented macro (before ylando did, a couple of days later).

2 points by rocketnia 541 days ago | link

"Isn't there a dialect out there that uses a different bracket to mean 'close all open parens'? So that the example above would become (foo (a b c (d e f]. I can't quite place the memory."

Oh, apparently it's Interlisp! They call the ] a super-parenthesis.

  The INTERLISP read program treats square brackets as 'super-parentheses': a
  right square bracket automatically supplies enough right parentheses to match
  back to the last left square bracket (in the expression being read), or if none
  has appeared, to match the first left parentheses,
  e.g.,    (A (B (C]=(A (B (C))),
           (A [B (C (D] E)=(A (B (C (D))) E).
Here's a document which goes over a variety of different notations (although the fact they say "there is no opening super-parenthesis in Lisp" seems to be inaccurate considering the above):

They favor this approach, which is also the one that best matches the way I intend for Parendown to work:

"Krauwer and des Tombe (1981) proposed _condensed labelled bracketing_ that can be defined as follows. Special brackets (here we use angle brackets) mark those initial and final branches that allow an omission of a bracket on one side in their realized markup. The omission is possible on the side where a normal bracket (square bracket) indicates, as a side-effect, the boundary of the phrase covered by the branch. For example, bracketing "[[A B] [C [D]]]" can be replaced with "[A B〉 〈C 〈D]" using this approach."

That approach includes what I would call a weak closing paren, 〉, but I've consciously left this out of Parendown. It isn't nearly as useful in a Lispy language (where lists usually begin with operator symbols, not lists), and the easiest way to add it in a left-to-right reader macro system like Racket's would be to replace the existing open paren syntax to anticipate and process these weak closing parens, rather than non-invasively extending Racket's syntax with one more macro.


1 point by akkartik 546 days ago | link

What are the semantics of `lets` exactly? Do you have to alternate binding forms with 'body' expressions?


2 points by rocketnia 546 days ago | link

No, you can interleave bindings and body expressions however you like, but the catch is that you can't use destructuring bindings since they look like expressions. It works like this:

  (lets) -> nil
  (lets a) -> a
  (lets a b . rest) ->
    If `a` is ssyntax or a non-symbol, we treat it as an expression:
      (do a (lets b . rest))
    Otherwise, we treat it as a variable to bind:
      (let a b (lets . rest))
The choice is almost forced in each case. It almost never makes sense to use an ssyntax symbol in a variable binding, and it almost never makes sense to discard the result of an expression that's just a variable name.

The implementation is here in Lathe's arc/utils.arc:

Current link:

Posterity link: