[aredington]

The example cited makes two decisions which are not articulated explicitly but are decisions one can make and which have benefits and costs. They are:

1) The function correctly executes on the class of things for which (.toString thing) has a sensible run time invocation.

2) The function delays examination of the correct class of its argument from compile time to run time.

1 is a benefit, in that in a dynamic system it is possible that something which, at compile time, does not have a sensible .toString invocation, can gain it at run time and then participate in the function.

2 is a cost, in that you have to delay classification to the instant in which you try to invoke .toString

This tradeoff is elemental and there will be systems you can build by embracing it that will never be possible in strongly typed languages. You'll be able to build isomorphs of them, but doing so will involve expressing significantly more ideas to get there.