cool. Is this preferable?

fun sql(colb : Int?) = "SELECT * FROM TABLE WHERE A=3 ${colb?.let {" AND B=$it"}.orEmpty()}"

Preferable in what sense? IMO including the last bit inside a string literal is an unnecessary complication

The point of templates seems to me that you don't need to use string concatenation. Your example was using a '+' in theory this could create more than one stringbuilder

Do you think that would be a bottle neck of your program?

Sorry about the slow response. The java compiler generates a StringBuilder also for + operations. My original point was that the code could in theory generated more than one StringBuilder for the whole expression instead of just the one. I realise it is not going to be a major bottleneck and I don't think performance should be a consideration in this example. However it feels to me that the whole point of templates was to get rid of the use of + as it is less readable.