Lists of maps with single item require trailing comma

[loilo]

I ran into a Sass compiler error recently with a combination of lists and maps.
What I have is a list containing a number of maps. This works without any problems until I only have a single map in that list.

At that point Sass requires a trailing comma after the list item, otherwise it will not be recognized as a map:

@use "sass:map";
@use "sass:list";

$list-of-maps: (
  (
    "key": "value"
  )
);

$list-item: list.nth($list-of-maps, 1);
$value: map.get($list-item, "key");

This produces the following error:

Error: $map: "key" "value" is not a map.

However, the code compiles correctly if a trailing comma is added after the list item:

@use "sass:map";
@use "sass:list";

$list-of-maps: (
  (
    "key": "value"
-  )
+  ),
);

$list-item: list.nth($list-of-maps, 1);
$value: map.get($list-item, "key");

I have created a minimal reproduction repo here.

[jathak]

The issue here is that (<expr>) is parsed as just <expr> wrapped in parentheses, rather than a single item list, so $list-of-maps is just a map without the trailing comma, rather than a list with one item.

To write a single-item list in Sass, you either need to add a trailing comma after the item to indicate that the parentheses are making a list, or you can use square brackets for your list. See the documentation on lists here.

[loilo]

Yes, I assumed that this had some kind of reasonable purpose, was mostly wondering if that edge case is documented somewhere. However, now it's findable via an issues search, which is something I guess. 😁

Thanks for the explanation!

[nex3]

In the second paragraph of the lists documentation it says:

A single-element list can be written either (<expression>,) or [<expression>]