[racket-dev] generic binding forms

On Sun, Aug 25, 2013 at 10:54 PM, Stephen Chang <stchang at ccs.neu.edu> wrote:
> Hi dev,
>
> I've noticed that Racket has a lot of convenient binding forms but
> they don't fit together unless someone does it manually (for example
> there's match-let and match-let-values, but no match-for).
>
> As an educational side project, I've been toying around with a
> different way of organizing all the binding forms. What I wanted to do
> is remove the need to manually combine current (and future) binding
> forms by moving the binding "logic" to the binding site itself.
>
> Inspired by the in-vogue generics movement in the Racket world, I've
> hacked together a sort of "generic interface" for bindings (in
> reality, it's just a bunch of syntax properties right now), and
> implemented alternate versions of some of the main binding forms that
> support "instances" of these generic bindings.
>
> To illustrate, here are some test cases for a generic define I
> implemented (~define). I also implemented binding "instances" for
> match and values (which I arbitrarily named $ and ~v below) and I can
> use these forms in (mostly) any binding position that supports generic
> bindings.
>
> ;; functions
>> (~define (f1 x y) (+ x y))
>> (f1 10 20)
> 30
>> (~define (f2 ($ (list x y))) (+ x y))
>> (f2 (list 10 20))
> 30
>
> ;; non-functions
>> (~define a1 100)
>> a1
> 100
>> (~define (~v a2 a3) (values 134 456))
>> a2
> 134
>> a3
> 456
>
> You can nest bind instances too:
>
>> (~define (~v ($ (list b1 b2)) b3) (values (list 22 33) 44))
>> b1
> 22
>> b2
> 33
>> b3
> 44
>
> Does anyone think this is useful? Or is it just a lot of work to save
> a little bit of typing? Has anyone tried something like this before?
>
> It's still very much a work in progress but I figure I would ask for
> some feedback earlier rather than too later, in case there is
> something that makes this infeasible.
>
> Brave souls can look at the hackery here:
> https://github.com/stchang/generic-bind/blob/master/generic-bind.rkt
> (Warning: I'm still trying to figure out all the toys in the Racket
> macro toolbox. For the most part, everything still looks like a
> syntax-rule/case/parse/->datum nail to my hammer.)
>
> Technical question: I couldn't figure out a nice way to implement
> ~let. Essentially, I want a let form where some clauses are let-values
> and some are match-let, but I need to bind them all at the same time,
> like let.

I'd introduce new names to bind them in sequence and then rename
everything after it's all available. My letwreck does something
similar:

http://jeapostrophe.github.io/2013-08-05-letwreck-post.html#%28elem._%28chunk._~3cletwreck-defn~3e~3a1%29%29

> I can't define a ~lambda that works with values because
> functions in racket can't receive values. Anyone have any ideas?
>
> Side observation: Trying to get things to work with multiple return
> values was a pain because they don't compose (as in, functions can
> produce them but can't receive them). Not sure if anything can be done
> about this though.
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-- 
Jay McCarthy <jay at cs.byu.edu>
Assistant Professor / Brigham Young University
http://faculty.cs.byu.edu/~jay

"The glory of God is Intelligence" - D&C 93