Communicating Haskell Processes

Concurrent programming supports multiple threads of concurrent execution. It

is a suitable paradigm for interaction with the outside world, where many inputs

(e.g. network packets, key-presses, mouse-clicks) may arrive at any time and need

to be dealt with at the same time. Concurrent execution also potentially allows

for parallel speed-up; multicore machines are now the standard for new PCs, but

the quest to take full advantage of the available parallelism continues.

One approach to concurrent programming is process-oriented programming,

which uses message-passing and is based on Hoare and Roscoe's Communicating

Sequential Processes (CSP). CSP is a process calculus centred on concurrent processes that communicate with each other via synchronous channels. This is used

as a formal underpinning for process-oriented programming, either directly (proving behaviours of programs with a CSP model checker) or indirectly (providing

sound design principles and patterns).

Process-oriented programming has previously been provided in programming

languages such as occam-? and libraries for other mainstream languages such as

Java and C++. However, as yet process-oriented programming has failed to gain

much traction via these implementations; occam-? is rooted in the much older

language occam and thus lacks powerful data structures (among other features)

which are standard in most modern languages { while languages such as Java

and C++, where data is mutable and easily shared, can be an awkward fit to

process-oriented programming.

Haskell is a functional programming language that is notable for its purity,

type-classes, monads and lazy evaluation: modern features that provide interest-

ing and powerful ways to program, including good support for imperative pro-

gramming. Like other functional languages, it eliminates mutable data -- which

immediately removes a whole class of problems in concurrent programming.

This thesis contends that CSP and process-oriented programming ?t well with

Haskell. The thesis details the creation of a CSP library for Haskell (Commu-

nicating Haskell Processes: CHP) { which features powerful support for process

composition { and its further augmentation with capabilities such as support for

a new concurrency primitive (conjunction), tracing and the ability to generate

formal models of CHP programs. This allows programmers to build concurrent

message-passing systems with a strong formal underpinning in a modern program-

ming language, with more features and less complications than process-oriented

libraries for other languages.