Regarding the last question: "what happens with yeti when you have polymorphism and mutable containers?"

In section:

http://linux.ee/~mzz/yeti/intro.html#type-declarations

"Variable (and argument) bindings are not polymorphic (it would make typesystem unsound), and therefore their type changes:"

For more details about this type unsoundness in ML, see the long version of "Relaxing the value restriction":

http://www.kurims.kyoto-u.ac.jp/preprint/file/RIMS1444.pdf