Re: Python slices in Scheme

[Jay Sulzberger]

On Sun, 18 Jun 2023, Damien Mattei <damien.mattei@gmail.com> wrote:

> hello,
>
> i'm porting the Python slicing (
> https://docs.python.org/2/reference/expressions.html#slicings ) to Scheme
> Guile and Racket.
>
> examples in Scheme+ :
>
> {#(1 2 3 4 5 6 7)[2 / 5]}
> #(3 4 5)
>
> i'm using / instead of : because : is already used by the SRFI 42 Eager
> Comprehension

Damien, I just now looked, but have not yet read carefully, the
Scheme+ manifesto.

Heaven forwarding, I will try some of the code before the snows come.

Thank you!

oo--JS.


> below are my testing examples:
>
> ;;; compiled
> /Users/mattei/.cache/guile/ccache/3.0-LE-8-4.6/usr/local/share/guile/site/3.0/Scheme+.scm.go
>
> scheme@(guile-user)> {#(1 2 3 4 5 6 7 8)[/ / 3]}
> $1 = #(1 4 7)
> scheme@(guile-user)> {#(1 2 3 4 5 6 7 8)[/ / -2]}
> $2 = #(8 6 4 2)
> scheme@(guile-user)> {"elephant"[2 / 5]}
> $3 = "eph"
> scheme@(guile-user)> {"abcdefghijkl"[/ / -3]}
> $4 = "lifc"
> scheme@(guile-user)> {"123456789"[ /  / -1]}
> $5 = "987654321"
> scheme@(guile-user)> {"abcdefghijkl"[/ / 2]}
> $6 = "acegik"
> scheme@(guile-user)> {#(1 2 3 4 5 6 7 8 9)[/ 7 / 2]}
> $7 = #(1 3 5 7)
> scheme@(guile-user)> {#(1 2 3 4 5 6 7 8 9)[/ 6 / -1]}
> $8 = #(6 5 4 3 2 1)
> scheme@(guile-user)> {#(1 2 3 4 5 6 7 8 9)[/ 6 / -2]}
> $9 = #(6 4 2)
> scheme@(guile-user)> {#(1 2 3 4 5 6 7 8 9)[/ -3 / -2]}
> $10 = #(6 4 2)
> scheme@(guile-user)> {#(1 2 3 4 5 6 7 8 9)[3 / / 2]}
> $11 = #(4 6 8)
> scheme@(guile-user)> {#(1 2 3 4 5 6 7 8 9)[3 / / -2]}
> $12 = #(4 2)
> scheme@(guile-user)> {#(1 2 3 4 5 6 7 8 9)[-3 / / 2]}
> $13 = #(7 9)
> scheme@(guile-user)> {#(1 2 3 4 5 6 7 8 9)[-3 / / -2]}
> $14 = #(7 5 3 1)
> scheme@(guile-user)> {"123456789"[/ -3 / -2]}
> $15 = "642"
> scheme@(guile-user)> {"abcdefghijklmno"[/ 7 / 2]}
> $16 = "aceg"
> scheme@(guile-user)> {"123456789"[/ -3 / -2]}
> $17 = "642"
> scheme@(guile-user)> {"abcdefghijklmno"[3 / / 2]}
> $18 = "dfhjln"
> scheme@(guile-user)> {"123456789"[3 /  / 2]}
> $19 = "468"
> scheme@(guile-user)> {"123456789"[3 /  / -2]}
> $20 = "42"
> scheme@(guile-user)> {"123456789"[-3 /  / -2]}
> $21 = "7531"
> scheme@(guile-user)> {"123456789"[-3 /  / 2]}
> $22 = "79"
> scheme@(guile-user)> {#(1 2 3 4 5 6 7 8 9)[2 / 5 / 1]}
> $23 = #(3 4 5)
> scheme@(guile-user)> {#(1 2 3 4 5 6 7 8 9)[5 / 2 / -1]}
> $24 = #(6 5 4)
> scheme@(guile-user)> {#(1 2 3 4 5 6 7 8 9)[2 / 5 / -1]}
> $25 = #()
> scheme@(guile-user)> {#(1 2 3 4 5 6 7 8 9)[-1 / 5 / -1]}
> $26 = #(9 8 7)
> scheme@(guile-user)> {#(1 2 3 4 5 6 7 8 9)[-0 / 5 / -1]}
> $27 = #()
> scheme@(guile-user)> {"0123456789"[5 / 2 / -1]}
> $28 = "543"
> scheme@(guile-user)> {"0123456789"[5 /  / -1]}
> $29 = "543210"
> scheme@(guile-user)> {"0123456789"[5 / 0 / -1]}
> $30 = "54321"
>
> the syntax is derived from Python syntax:
>
> [1,2,3,4,5,6,7,8,9][-3::2]
> [7, 9]
> [1,2,3,4,5,6,7,8,9][-3::-2]
> [7, 5, 3, 1]
> [1,2,3,4,5,6,7,8,9][3::-2]
> [4, 2]
> "abcdefghijkl"[: : -3]
> 'lifc'
> [1,2,3,4,5,6,7,8,9][:5:-1]
> [9, 8, 7]
> [1,2,3,4,5,6,7,8,9][:5:1]
> [1, 2, 3, 4, 5]
> [1,2,3,4,5,6,7,8,9][0:5:-1]
> []
> [1,2,3,4,5,6,7,8,9][-0:5:-1]
> []
> [1,2,3,4,5,6,7,8,9][-1:5:-1]
> [9, 8, 7]
> [1,2,3,4,5,6,7,8,9][:5:-1]
> [9, 8, 7]
> [1,2,3,4,5,6,7,8,9][10:5:-1]
> [9, 8, 7]
> [1,2,3,4,5,6,7,8,9][2:5:-1]
> []
> [1,2,3,4,5,6,7,8,9][5:2:-1]
> [6, 5, 4]
> [0,1,2,3,4,5,6,7,8,9][5:2:-1]
> [5, 4, 3]
> [0,1,2,3,4,5,6,7,8,9][5::-1]
> [5, 4, 3, 2, 1, 0]
> [0,1,2,3,4,5,6,7,8,9][5:0:-1]
> [5, 4, 3, 2, 1]
>
> for now it works in simple expressions,soon it will be implemented for
> expressions with LHS and RHS like {container1[2 / 5] <- container2[7 / 9]}
> for this reason all will be in the next version of Scheme+
> https://github.com/damien-mattei/Scheme-PLUS-for-Guile
> with other features too.
>
> Damien