[throwaway81523]

Ben Lippmeier's Disciplined Disciple Compiler (DDC), for his language later called Discus, was interesting. It was/is an experimental language that managed mutation through an effect typing system. In the intro to his thesis he talks about it some.

Discus language: http://discus-lang.org/

Thesis: https://benl.ouroborus.net/papers/2010-impure/lippmeier-impu...

The thesis is the more interesting of those two links IMHO. The intro is chapter 1 that starts at page 17 of the pdf. It has one of the better critiques of Haskell that I've seen, and explains why uncontrolled mutation is not the answer. Reference types ala ML aren't the answer either, in his view.