Macros

Armed with an understanding of symbols and expressions, we can now discuss creating macros.

A macro in Julia constructs boilerplate code by substituting its argument(s) after parsing and has no knowledge of the argument values. However, this is a great improvement to macros in (say) C/C++, which are essentially merely preprocessors prior to the parsing phase.

The first macro is very simple – if an expression is passed, it prints out its argument list; otherwise, it just returns:

# Check 'ex' is an expression, else just return it.
julia> macro pout(ex)
  if typeof(ex) == Expr
    println(ex.args)
  end
  return ex
end
julia> x = 1.1; @pout x
1.1
# For an expression return its arguments and then evaluate it.
julia> y = 2.3;
julia> @pout (x^2 + y^2 - 2*x*y)^0.5
Any[:^, :((x ^ 2 + y ^ 2) - 2 * x * y), 0.5]
1.1999999999999997

The following is a slightly more complex macro, which executes a body...