Skip to content

Add list slice #706

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
annevk merged 5 commits into from
Apr 14, 2026
Merged

Add list slice #706

Changes from 4 commits
Commits
Show all changes
5 commits
Select commit Hold shift + click to select a range
dfe9571
Add slice
jakearchibald Apr 9, 2026
ce2df87
Restore note
jakearchibald Apr 9, 2026
816b8b4
Expand slice algorithm
jakearchibald Apr 10, 2026
5dfaeb9
Handle out of bound values
jakearchibald Apr 10, 2026
f6c8e61
Asserts rather than clamping
jakearchibald Apr 14, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
28 changes: 24 additions & 4 deletions infra.bs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1661,10 +1661,30 @@ set of steps on each <a for=list>item</a> in order, use phrasing of the form
"<a for=list>For each</a> |item| of <var ignore>list</var>", and then operate on |item| in the
subsequent prose.

<p>To <dfn export for=list,stack,queue,set>clone</dfn> a <a>list</a> |list| is to create a new
<a>list</a> |clone|, of the same designation, and, <a for=list>for each</a> |item| of |list|,
<a for=list>append</a> |item| to |clone|, so that |clone| <a for=list>contains</a> the same
<a for=list>items</a>, in the same order as |list|.
<div algorithm=slice>
<p>To <dfn export for=list,stack,queue,set>slice</dfn> a <a>list</a> |list|, with an optional
integer <dfn export for=list/slice><var>from</var></dfn> (default 0) and an optional integer
<dfn export for=list/slice><var>to</var></dfn> (default |list|'s <a for=list>size</a>), perform the
following steps. They return a <a>list</a> of the same designation as |list|.

<ol>
<li><p><a>Assert</a>: |to| is greater than or equal to |from|.

<li><p>If |from| is less than 0, then set |from| to 0.

<li><p>If |to| is greater than |list|'s <a for=list>size</a>, then set |to| to |list|'s <a for=list>size</a>.
Copy link
Copy Markdown
Contributor Author

@jakearchibald jakearchibald Apr 10, 2026 •
edited
Loading

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I felt this was preferable to asserting. It means, like in JS, you can slice from 0 to 100 to 'crop' a list to maximum of 100 items.

['foo'].slice(0, 100); // does not throw

Copy link
Copy Markdown
Member

@annevk annevk Apr 10, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Is this needed for specifications? I might not be considering all scenarios that well, but I was assuming we'd just want to require both of these to be within bounds of the list.

Copy link
Copy Markdown
Contributor Author

@jakearchibald jakearchibald Apr 14, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I don't mind turning these into asserts. I was just copying how JS array.slice() works.

Copy link
Copy Markdown
Member

@annevk annevk Apr 14, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Let's start out with a very strict contract then until we hit cases where it would make sense to not be so strict.

Copy link
Copy Markdown
Contributor Author

@jakearchibald jakearchibald Apr 14, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Done!


<li><p>Let |slice| be a new <a>list</a> of the same designation as |list|.

<li><p><a for=set>For each</a> |i| of <a lt="the exclusive range">the range</a> |from| to |to|,
exclusive: <a for=list>append</a> |list|[|i|] to |slice|.

<li><p>Return |slice|.
</ol>
</div>

<p>To <dfn export for=list,stack,queue,set>clone</dfn> a <a>list</a> |list| is to return a
<a for=list>slice</a> of |list|.

<p class=note>This is a "shallow clone", as the <a for=list>items</a> themselves are not cloned in
any way.
Expand Down