15-212: Principles of Programming

The programming techniques used so far in the course have, for the most part, been "purely functional". Some problems, however, are more naturally addressed by keeping track of the "state" of an internal machine. Typically this requires the use of mutable storage. ML supports mutable cells, or references, that store values of a fixed type. The value in a mutable cell can be initialized, read, and changed (mutated), and these operations result in effects that change the store. Programming with references is often carried out with the help of imperative techniques. Imperative functions are used primarily for the way they change storage, rather than for their return values.

Key Concepts

• Mutable cell
• Storage effect
• Reference
• Aliasing

Sample Code

Handouts

• Some Notes on Mutable References

On-line Notes by Robert Harper

• References

Suggested Reading in Programming in Standard ML:

• Chapter 13: Mutable Storage

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Michael Erdmann