2.9: Mutable references

As noted previously, everything in Standard ML is immutable by default. val gives a name to a value. This is most commonly how SML is used, and mutability is rare. It may be useful sometimes to have a mutable reference, so SML does provide the capability to do so.

You may define a reference using ref, get its value using ! before the name, and set it using :=.

Functions may accept references as parameters, and this is reflected in the type.