I think it is some ugly, more readable (with spaces):

#{map-get($grid-breakpoints, "sm") - 1}

Sure will make the changes

Can we add couple test more? the harder is the better

sure, I will try to add. Do you have anything in mind?

Something difficult with sass maps/list with interpolation

added new testcases

Bug?

I guess no. checked in the playground. Over there also it is formatting in the same way link

Let's add/fix two cases here in:

.text-#{ $name }

and

.text #{ $name }

sorry, didn't get your point. Can you please explain a bit.
.text-
#{ $name }
is formatting into
.text- #{ $name }

do you think it a bug? because we can't just assume that if it ends with "-", we have to merge the next text without space. The class name can also be just "test-", right?

No, it is not bug, just put in tests:

@each 
$size 
in 
$sizes {
.icon
#{$size} 
{
border
: 
"#{$size + "px"}" 
solid 
red;
}
}

and

@each 
$size 
in 
$sizes {
.icon-#{$size} 
{
border
: 
"#{$size + "px"}" 
solid 
red;
}
}

and

//map test
$font-weights
: 
("regular": 400
, 
"medium": 500, 
"bold": 
700);
@each 
$name, 
$boldness 
in 
$icons {
.text
#{
$name
} {
color: red;
font-weight: 
"#{$boldness}"
}
}

//map test
$font-weights
: 
("regular": 400
, 
"medium": 500, 
"bold": 
700);
@each 
$name, 
$boldness 
in 
$icons {
.text-#{
$name
} {
color: red;
font-weight: 
"#{$boldness}"
}
}

Instead

Thanks, will make the changes

Not sure it is safe to trim here, why we do it?

Stringify was adding extra '\n', so I am using trim

.icon-#{$size} above didn't add space around

@fisker and should not, you can write it in different ways (all valid):

.text foo { }

.text- foo {}

.text-foo {}

I mean space inside #{}, this one has space around $name, but .icon-#{$size} don't.

Yep, you are right

/cc @agamkrbit

I well make the changes ..
so .icon-#{$size} should be like .icon-#{ $size } right ?

Looks like we are keeping spaces on stable version.

Prettier 2.1.2
Playground link

--parser scss

Input:

#{$a                      } {
    color: red;
  }

#{                      $a} {
    color: red;
  }
#{           $a           } {
    color: red;
  }
#{$a} {
    color: red;
  }

Output:

#{$a } {
  color: red;
}

#{ $a} {
  color: red;
}
#{ $a } {
  color: red;
}
#{$a} {
  color: red;
}

yeah.