Arguably the simplest notation for string concatenation would be just concatenation (which would be the notation for multiplication, hence why Julia's choice could be argued to eb the most natural). Unfortunately programming languages can't quite deal with the ambiguity of having `a (x + y)` and `f(x)` refer to different operations.

Some languages seem to allow it for string literals though.

In Julia, you can actually omit the `*` for multiplication, if it involves literals, i.e.

  julia> x = 10
  10
  julia> 2x
  20
  julia> 2 x
  ERROR: syntax: extra token "x" after end of expression

But you have to be careful not to use white space either.

That’s quite clever, actually. This way you could chain a complex string together with parentheses without needing a lot of pluses.

Not the most useful of features, but it does explain why * is the concat operator.

Well now I want a language to overload white space as an operator

C++2000 has you covered: http://www.stroustrup.com/whitespace98.pdf

But that doesn't help, because * also commutes, and string concatenation still doesn't.

> * also commutes

It does for numbers, but not generally - e.g. for matrices.