> On Jul 11, 2017, at 12:38 AM, Nadeem Abdul Hamid <nad...@acm.org> wrote:
>
> ... though a syntactic solution combining for and match would probably
> be better.
>
> Yes, please!...
> Look, python can do it:
>
> >>> [(x, z) for (x, y, z) in [(1, 2, 3), (4, 5, 6), (7, 8, 9), ('a', 'b',
> >>> 'c')]]
> [(1, 3), (4, 6), (7, 9), ('a', 'c')]
>
> :-)
You can do this with the Generic Bind package [1]. It's goal is to allow you to
use patterns like this in any binding form that accepts generic binding
instances, including an extended version of for/list.
raco pkg install generic-bind
#lang racket
(require generic-bind)
(~for/list ([($list x y z) '((1 2 3) (4 5 6) (7 8 9) (a b c))])
(list x z))
;'((1 3) (4 6) (7 9) (a c))
If you want to use the name `for/list` without the `~`, you can require
`generic-bind/as-rkt-names` instead.
#lang racket
(require generic-bind/as-rkt-names)
(for/list ([($list x y z) '((1 2 3) (4 5 6) (7 8 9) (a b c))])
(list x z))
;'((1 3) (4 6) (7 9) (a c))
It also allows arbitrary match patterns using the `$` form, so `($ (list x y
z))` would work the same way.
[1]: https://docs.racket-lang.org/generic-bind/index.html
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