EBNF: interpolate
var = "${" [^}]+ "}" => [[ [ second >string search , [ present write ] % ] [ ] make ]]
text = [^$]+ => [[ [ >string , [ write ] % ] [ ] make ]]
interpolate = (var|text)* => [[ [ ] join ]]
;EBNF

: I[ "]I" parse-multiline-string interpolate parsed \ call parsed ; parsing

! Example
:: foo ( a b -- ) I[ Hello ${a} and ${b}]I ;

"foo" "bar" foo