I had the same reaction initially but then realized that this makes the variable declaration / initialization syntax very consistent.

  variable: Type [= initial_value];

  // initializes foo to an int with an initial value of 43 
  foo: int = 43;

  // initializes main to a (presumably, as I haven't read the
  // spec yet, const)pointer to a function that takes no 
  // arguments and returns an int.
  main: () -> int = {
     ...
  }