This is a simple example of using the ST Monad for stateful summation of a list.

The ST monad in Haskell allows you to perform mutable (stateful) computations in a safe, local way. It stands for "State Thread" and is defined in Control.Monad.ST (ST).

Key Points

Local Mutability

You can create and modify mutable variables (STRefs) inside the ST monad.

Safety

Changes made in the ST monad are not visible outside; the mutation is encapsulated.

Pure Interface

When you run an ST computation (using runST), you get a pure result, and the mutable state cannot escape.

Example usage

Computes the sum of a list using the mySum function:

let total = mySum [1, 2, 3, 4, 5]