One answer would be: So that when you hit the ( you know it's a list of parameters and not an expression, even before you spot the -> I still haven't read enou

One answer would be: So that when you hit the ( you know it's a list of parameters and not an expression, even before you spot the -> I still haven't read enough about syntax 2 to understand if it allows any other use of ( after a : but if yes, a keyword like fn or fun would make the parser much more regular.