Joyce (programming language)
|Paradigm(s)||concurrent, imperative, structured|
|Designed by||Brinch Hansen|
|Stable release||1 / 1987|
|Influenced by||Communicating Sequential Processes, Pascal, Concurrent Pascal|
Joyce is a secure, concurrent programming language designed by Per Brinch Hansen in the 1980s. It is based on the sequential language Pascal and the principles of Communicating Sequential Processes (CSP). It was created to address the shortcomings of CSP to be applied itself as a programming language, and to provide a tool, primarily for teaching, for distributed system implementation.
The language is based around the concept of agents; concurrently executed pocesses that communicate only by the use of channels and message passing. Agents may activate sub-agents dynamically and recursively. The development of Joyce formed the foundation of the language SuperPascal, also developed by Brinch Hansen around 1993.
Joyce is based on a small subset of Pascal, extended with features inspired from CSP for concurrency. The following sections describe the some of the more novel features that were introduced.
An agent is a procedure consisting of a set of statements and possibly nested definitions of other agents. An agent may dynamically activate sub-agents which execute concurrently with their creator. An agent can terminate only when all of its sub-agents have also terminated. For example, an agent
agent process1(x, y: integer); begin ... end; agent process2(); use process1; begin process1(9, 17); end;
The activation of an agent creates new instances of all local variables and the value of each formal parameter is copied to a local variable. Hence, agents cannot access variables of other agents and are allowed only to communicate through the use of channels. This restriction prevents problems associated with the use of shared variables such as race conditions.
Agents communicate through entities called channels. Channels have an alphabet, defining the set of symbols which may be transmitted. Channels are created dynamically and accessed through the use of port variables. A port type is defined by a distinct set of symbols constituting its alphabet. Symbols with multiple values are defined with a specific type. For example:
stream = [int(integer), eos];
int(integer) denotes a message symbol called
int of any integer value. The second typeless symbol declaration
eos (end of stream) is known as a signal. Once a port type has been defined, a port variable of that type can be declared:
out : stream in : stream
And then a channel entity, internal to the agent creating it, can be activated as follows:
Symbols can then be sent and received on channels using the CSP-style input and output operators
! respectively. A communication can only occur if there is a receiving agent matching the sending agent. The receiving agent must expect to receive the symbol type being sent. For example, the value 9 followed by the
eos symbol is sent on port
out ! int(9) out ! eos
And an integer message is received into a variable of a matching type, followed by the
received : integer in ? int(received) in ? eos
Polling statements are based the CSP concept of guarded alternatives. A polling statement is made up of a set of statements, each guarded by an input channel statement. When a communication is matched between a transmitting agent and a guard, the guard is executed, followed by the corresponding statement. For example:
poll in ? X -> x := x + 1 | in ? Y -> y := y + 1 end
Where the port
in is monitored for the signals
Y, on a matching communication, the corresponding variables
y are incremented.
Joyce was designed to be a secure language in the sense that a compiler would be able to detect all violations of the language rules.
The following is a complete example program, taken from the original paper introducing the Joyce programming language, implementing an algorithm to generate prime numbers based on a sieving technique. A
sieve agent is sent a stream of integers from its predecessor, the first being a prime. It removes all multiples of this prime from the stream and activates a successor. This continues until the
eos signal is propagated along the set of sieves.
agent sieve(inp, out: stream); var more: boolean; x, y: integer; succ: stream; begin poll inp?int(x) -> +succ; sieve(succ, out); more := true | inp?eos -> out!eos; more := false end; while more do poll inp?int(y) -> if y mod x <> 0 then succ!int(y) | inp?eos -> out!int(x); succ!eos; more := false end; end;
The following agent initialises the set of sieve agents and inputs into them a stream of integers between 3 and 9999.
agent primes; use generate, sieve, print; var a, b: stream; begin +a; +b; generate(a, 3, 2, 4999); sieve(a, b); print(b) end;
Due to concurrent execution of agent procedures, a conventional sequential stack allocation scheme cannot be used as the activation records of the agent calls do not follow a last-in first-out pattern. Instead, the creator-subagent relationships form a tree-structured stack. A simple scheme is used to implement this behaviour, which works by allocating new activation records at the top of the stack, and linking subagents' activation records to their creator's record. These records are freed only when the agent has terminated and they are at the top of the stack. The effectiveness of this scheme depends on the structure and behaviour of a program, which in some cases will result in poor memory usage. A more effective scheme was implemented in the SuperPascal language.
- Per Brinch Hansen, Joyce--A programming language for distributed systems
- Per Brinch Hansen, The Joyce language report, Software--Practice and experience, Vol. 19(6), 553-578 (June 1989)
- Per Brinch Hansen, A multiprocessor implementation of Joyce. Software -- Practice and experience 19, 6 (June 1989), 579-592
- Per Brinch Hansen Archive - A collection of Brinch Hansen's papers.