How does functional programming support concurrent programming?

Functional programming supports concurrent programming by avoiding mutable state and side effects, which simplifies concurrent code.

Functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data. This is in contrast to imperative programming where code is composed of statements which change global state when executed. Functional programming is designed on the concept of immutability, which makes it naturally suitable for concurrent programming.

In concurrent programming, multiple computations are executing during overlapping time periods—potentially interacting with each other. The main challenge in concurrent programming is coordinating access to shared state. If one computation is updating a shared variable while another is reading from it, the program could end up in an inconsistent state.

However, in functional programming, data is immutable, meaning once a data structure is created, it cannot be changed. Any 'change' to a data structure results in a new data structure. This eliminates the possibility of race conditions, deadlocks and other synchronisation problems associated with shared mutable state. This makes it easier to write safe concurrent programs.

Moreover, functional programming languages often include powerful abstractions for managing concurrency and parallelism. For example, Haskell has lightweight threads and software transactional memory, and Erlang has message-passing concurrency built into the language. These features make it easier to reason about and write concurrent code.

In addition, functional programming supports higher-order functions and lazy evaluation. Higher-order functions allow you to abstract common patterns of computation, making your code more modular and easier to reason about. Lazy evaluation allows you to create potentially infinite data structures, which can be very useful in concurrent programming.

In conclusion, functional programming's emphasis on immutability, lack of side effects, and powerful abstractions make it well-suited to concurrent programming. By eliminating many of the common pitfalls of concurrent programming, functional programming allows you to write concurrent code that is easier to understand, reason about, and debug.