UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science

Multilisp: Concurrent Functional Programming

Ed Walters and Tim Richards

University of Massachusetts Amherst

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 2

Overview What is Multilisp? Overview of Scheme Features of Multilisp Where is Multilisp Now?

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 3

Overview What is Multilisp? Overview of Scheme Features of Multilisp Where is Multilisp Now?

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 4

What is Multilisp? Dialect of Scheme language

Functional Programming Limited Side-effects Garbage Collection

Extensions for Concurrency Parallel Function Application Futures

Reference: R. Halstead, TOPLAS, Vol. 7, No. 4, 1985.

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 5

Goal of Multilisp Explicit Constructs for Concurrency Adhere to Scheme Philosophy

No Additional Syntax Minimal Additional Semantics Maximum Flexibility

Granularity Backwards-compatibility

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 6

Overview What is Multilisp? Overview of Scheme Features of Multilisp Where is Multilisp Now?

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 7

The Scheme Language

Descendent of Lisp (LISt Processing)

Created by Steele and Sussman (1975)

Important Features: Extended Lambda Calculus Lexical Scoping Functions are first-class

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 8

Important Terms Expression: Basic unit of Scheme

code (e.g., List or integer) Evaluation: Scheme expressions

evaluate to a value upon execution Application: Function call on a list,

i.e. apply first element to rest of list (f a b c) => f(a, b, c)

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 9

Brief Scheme Syntax Function definition

(define f (lambda (x) … )) Function application

(f x) Conditionals

(if x y z) Symbols and Atomic Elements

‘x, 3

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 10

Scheme Example: Fib

(define fib (lambda (x)

(if (= x 1) 1(if (= x 2) 2

(+ (fib (- x 1)) (fib (- x 2)))))))

(fib 10) => 89

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 11

Scheme Example: Fib

(define fib (lambda (x)

(if (= x 1) 1(if (= x 2) 2

(+ (fib (- x 1)) (fib (- x 2)))))))

(fib 10) => 89

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 12

Scheme Example: Fib

(define fib (lambda (x)

(if (= x 1) 1(if (= x 2) 2

(+ (fib (- x 1)) (fib (- x 2)))))))

(fib 10) => 89

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 13

Scheme Example: Fib

(define fib (lambda (x)

(if (= x 1) 1(if (= x 2) 2

(+ (fib (- x 1)) (fib (- x 2)))))))

(fib 10) => 89

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 14

Scheme Example: Fib

(define fib (lambda (x)

(if (= x 1) 1(if (= x 2) 2

(+ (fib (- x 1)) (fib (- x 2)))))))

(fib 10) => 89

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 15

Scheme Example: Fib

(define fib (lambda (x)

(if (= x 1) 1(if (= x 2) 2

(+ (fib (- x 1)) (fib (- x 2)))))))

(fib 10) => 89

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 16

Scheme Example: Fib

(define fib (lambda (x)

(if (= x 1) 1(if (= x 2) 2

(+ (fib (- x 1)) (fib (- x 2)))))))

(fib 10) => 89

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 17

Overview What is Multilisp? Overview of Scheme Features of Multilisp Where is Multilisp Now?

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 18

Parallel Calls pcall: Parallel function application Syntax: (pcall F A B C) Semantics:

Evaluate F, A, B, C in parallel Apply F to A, B, C

(F A B C)

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 19

Parallel Calls: Example

(define div-and-conq

(lambda (x)

(if (base-case x) x

(pcall combine-results

(div-and-conq (- x 1))

(div-and-conq (- x 5))))))

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 20

Parallel Calls: Example

(define div-and-conq

(lambda (x)

(if (base-case x) x

(pcall combine-results

(div-and-conq (- x 1))

(div-and-conq (- x 5))))))

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 21

Parallel Calls: Example

(define div-and-conq

(lambda (x)

(if (base-case x) x

(pcall combine-results

(div-and-conq (- x 1))

(div-and-conq (- x 5))))))

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 22

Parallel Calls: Example

(define div-and-conq

(lambda (x)

(if (base-case x) x

(pcall combine-results

(div-and-conq (- x 1))

(div-and-conq (- x 5))))))

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 23

Parallel Calls: Example

(define div-and-conq

(lambda (x)

(if (base-case x) x

(pcall combine-results

(div-and-conq (- x 1))

(div-and-conq (- x 5))))))

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 24

Futures future: contract to deliver parallel

computation Syntax: (future <exp>) Semantics:

Evaluate <exp> concurrently with calling program

Return reference to future immediately

Block if value of <exp> is required

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 25

Futures: Example(define fib

(lambda (x) (if (= x 1) 1

(if (= x 2) 2(+ (future (fib (- x 1))) (future (fib (- x 2)))) ))))

(fib 10) => 89

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 26

Futures: Example(define fib

(lambda (x) (if (= x 1) 1

(if (= x 2) 2(+ (future (fib (- x 1))) (future (fib (- x 2)))) ))))

(fib 10) => 89

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 27

Futures: Example(define fib

(lambda (x) (if (= x 1) 1

(if (= x 2) 2(+ (future (fib (- x 1))) (future (fib (- x 2)))) ))))

(fib 10) => 89

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 28

Futures: Example(define fib

(lambda (x) (if (= x 1) 1

(if (= x 2) 2(+ (future (fib (- x 1))) (future (fib (- x 2)))) ))))

(fib 10) => 89

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 29

Futures: Example(define fib

(lambda (x) (if (= x 1) 1

(if (= x 2) 2(+ (future (fib (- x 1))) (future (fib (- x 2)))) ))))

(fib 10) => 89

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 30

More Details pcall can be implemented using future Facilitates multiple concurrent

programming paradigms: Cilk-style Message Passing

Futures somewhat resemble Lazy Evaluation Evaluation delayed but guaranteed No infinite data structures

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 31

Implementation Notes

Each future/pcall element is a thread

pcall fork/join parallelism

future asynchronous thread block on read

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 32

Overview What is Multilisp? Overview of Scheme Features of Multilisp Where is Multilisp Now?

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 33

Where is Multilisp Now?

No current Scheme compilers implement futures We had to implement our own interpreter!

However: THEY LIVE on in Java! Transparent Proxies for Java Futures,

Pratikakis, Spacco, and Hicks, OOPSLA 2004.

Safe Futures for Java, Welc, Jagannathan, and Hosking, OOPSLA 2005.

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTSASSACHUSETTS, A, AMHERST • MHERST • Department of Computer Science Department of Computer Science 34

Conclusion Like much of the Scheme World:

Elegant, Flexible Solution Deader than a Doornail Not Java