From Matz blog ( 2005-01-04, 2005-01-07, 2005-01-20 ).

Matz introduced and commented on Guido’s blog articles about Optional Static Typing on Python ( 1, 2, 3, 4 ) in these days. He talked up the early idea than latest one.

On Ruby, he made up his mind NOT to add Static Typing. Because he thinks the “optional” one is down by half and the “full” one lead to losing Ruby’s identity by dynamic language, even if it’s friendly for dynamic language. Besides, He concerned about its complexity, implements and performance as against its effect.

But he is ready to add Type Checking System if there is a good idea. He thinks that it’ll be the one like current C Extention’s type checking. He has an interest in friendly Duck-Typing, Object Adaptation, Design by Contract.