_ does not work in string interpolator

[scf37]

Compiler version

3.1.3

Minimized code

Seq(1, 2, 3).map(s"$_ ")

Output

error in interpolated string: identifier, `this' or block expected

Expectation

Successful compilation

Seq(1, 2, 3).map(s"${_} ") yields different error:
Unbound placeholder parameter; incorrect use of _

[dwijnand]

There's history here: scala/scala#2793 (comment)

[scf37]

Use case: Kotlin has very handy it keyword for functions with single anonymous parameter: values.map { "\"$it\"" }

It could be nice to have consistent _ in Scala: values.map(s"\"$_\"")

[som-snytt]

I agree it would be nice, and I would have used the feature a few times over the years.

The specification is straightforward, as it is ordinary placeholder syntax.

Let me check:

def f[A](xs: Seq[A]) = xs.map("prefix" + _)
def f[A](xs: Seq[A]) = xs.map("prefix" + s"$_")
def f[A](xs: Seq[A]) = xs.map("prefix" + StringContext("", "").s(_))

which would normally be written

def f[A](xs: Seq[A]) = xs.map(s"prefix$_")