Difference between State, ST, IORef, and MVar
问题 I am working through Write Yourself a Scheme in 48 Hours (I'm up to about 85hrs) and I've gotten to the part about Adding Variables and Assignments. There is a big conceptual jump in this chapter, and I wish it had been done in two steps with a good refactoring in between rather then jumping at straight to the final solution. Anyway… I've gotten lost with a number of different classes that seem to serve the same purpose: State , ST , IORef , and MVar . The first three are mentioned in the