Effective Haskell

Working with IORefs

In Haskell, rather than storing a value that is directly mutable, we store an immutable reference to some data that we can change using the reference. There are several different types of mutable references that you can use, depending on the specific needs of your application. You’ll learn about several different types of references throughout this chapter, but for the moment we’ll focus on particular type of reference, the IORef.

A value with the type IORef a is a normal Haskell value that holds an immutable pointer to some internal data that is managed by the GHC runtime.

You can use an IORef like any other normal Haskell value. You can pass it into a function, return it, store it in a record, or capture the value in a ...