Perl 6 does it without the sigil outside the bracket :)

"I have {some-complex-calc($i)} frobitzes"

Unlike Python's `"I have {} apples".format(n)` and various shells' `"I have $n apples"`, Javascript's and Apache Groovy's `"This is chapter ${i+1}"` mean the lexer must make a call to the parser when lexing the string contents -- this allows `"This is ${"chapter"} ${i+1}"` to parse. Don't know about Javascript, but this makes Groovy's parser unreadable, and stuck on a very old version of Antlr, i.e. 2 instead of 4 (the latest).

Python added https://www.python.org/dev/peps/pep-0498/.

Also in groovy. I too am a fan.