bjam acting weird?

Question

I thought the output of this code would be

Hello World 1
Hello World 2 3

But I get

Hello World 1
Hello World 2 Hello World 3

Here's the jam code

rule weird ( a : b + : c * : d ? )
{
  echo "Hello World $(a)" ;
  echo "Hello World $(b)" ;
}

weird 1 : 2 3 ;

Answer 1

Jam's variable expansion rules are very different from other languages. From the boost jam documentation:

The result of a token after variable expansion is the product of the components of the token, where each component is a literal substring or a list substituting a variable reference.

So jam is expanding "Hello World $(b)" to "Hello World $(b[1])" "Hello World$(b[2])" and echoing that.

If you want "Hello World 2 3", you need to split the echo argument into two tokens

echo "Hello World" $(b) ;