Very useful feature of many programming languages is string interpolation. It is a great syntax for concise string formatting. Interpolated string is nothing more than a template to be used to output formatted text. And it is a primary responsibility of the interpolation feature. Scala supports two modes of interpolation: value and expression.

val name = "Ozzy"
val surname = "Osbourne"
// value interpolation
val fullname = s"$name $surname"
// expression interpolation
val lengthMessage = s"Number of letters in the name ${name.length}"

And this is where I like Scala’s syntax a lot - to eliminate parsing ambiguity, expressions need to be enclosed by braces, which makes interpolated string look very ugly. And when something is ugly in the code, it makes me think what’s wrong with it. Well, there’s a problem - all expressions have their own responsibilities already. In the example above - expression is used to calculate name’s length, which is a separate responsibility on its own and placing length calculation into template is violation of Single Responsibility Principle.

While I don’t want anyone to blindly follow this rule of not using expression interpolation in your code, in general I believe it’s a good habit to prefer value interpolation over expression. It will make the code more readable without those distracting curly braces seeded in strings. And this gets to a clearer separation between responsibilities in the code. If you let expressions creep into the code, they may end up as hard to find side effects (calling external APIs, invoking expensive refresh of a cache).

Scala allows to put values into blocks too, but why would you do that? Don’t upset people who read your code.