Haskell provides built-in support for Software Transactional Memory (STM) as a mechanism for managing concurrent computations. STM helps ensure the correctness and consistency of shared state by allowing multiple threads to perform transactions atomically. Here's how Haskell handles concurrency using STM:

1. STM Monad :
   * STM is based on the `STM` monad, which provides a transactional context for performing concurrent operations on shared state.
   * Computations within the `STM` monad are composed using monadic operations like `>>=` and `return`.
   * The `Control.Concurrent.STM` module provides the necessary functions and types for working with STM in Haskell.

2. Transactional Variables :
   * STM operates on transactional variables called `TVar`s, which are mutable variables specifically designed for use in STM transactions.
   * `TVar`s are created using the `newTVar` function and can store any type that is an instance of the `Eq` and `Show` typeclasses.
   * Multiple threads can read and write to `TVar`s within a transaction, ensuring atomicity and consistency. 3. Atomic Transactions :
   * STM allows you to define atomic transactions using the `atomically` function. An atomic transaction groups a sequence of operations that should be executed atomically.
   * Within an atomic transaction, you can read the value of a `TVar` using `readTVar` and modify its value using `writeTVar`.
   * Transactions automatically roll back if any conflicts or inconsistencies occur during their execution.

4. Transactional Consistency :
   * STM provides transactional consistency, ensuring that concurrent transactions do not interfere with each other.
   * If two transactions attempt to modify the same `TVar` simultaneously, one of them will be retried (rolled back and retried later) to avoid conflicts.
   * Transactions that are retried do not block the executing thread; instead, they wait for the system to signal that the transaction can be retried.

5. Error Handling and Composition :
   * STM allows for error handling within transactions using functions like `catchSTM` and `retry`. If an exception is thrown within a transaction, it can be caught and handled gracefully.
   * Transactions can be composed using combinators like `orElse` and `orElseRetry`, allowing for conditional branching and composition of transactional logic.