delimited continuations for prolog - arizona state...

Delimited Continuations for Prolog

T. Schrijvers, B. Demoen, B. Desouter and J. Wielemaker

UGent, KU Leuven, UGent and Universiteit Amsterdam

Motivation

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Programming patterns

Minimality

Prolog = Horn clauses + simple builtins

⇒ Lack of infrastructure to capture common patterns

Solutions: meta-programming & program transformations

I Definite clause grammars (DCGs)

I Extended DCGs

I Structured state threading

I . . .

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Issues with non-local program transformations

1. O (n2) effort

2. Fragile w.r.t. evolution

3. New feature → impact entire system

Introduce DCGs in an existing code base to pass informationthrough several layers.

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Our solution: delimited continuations

I Program-level definitions

I More light-weight

I More robust

I No pervasive changes

M. Felleisen, The theory and practice of first-class prompts, ’88O. Danvy & A. Filinski, Abstracting control, ’90

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Outline

1. Definition

2. Applications

3. Implementation in the WAM

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Definition

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Delimited continuation primitives

p :−q ,w r i t e l n ( c ) .

q :−w r i t e l n ( a ) ,w r i t e l n ( b ) .

?− p .abc

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reset /3 and shift /1

p :−reset( q , , ) ,w r i t e l n ( c ) .

q :−w r i t e l n ( a ) ,shift( ) ,

writeln(b) . ← Continuation

?− p .ac

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p :−r e s e t ( q , , Term ) ,writeln(Term) ,w r i t e l n ( c ) .

q :−w r i t e l n ( a ) ,s h i f t (hello ) ,w r i t e l n ( b ) .

?− p .ahelloc

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p :−r e s e t ( q , Cont , ) ,w r i t e l n ( c ) ,call(Cont) .

q :−w r i t e l n ( a ) ,s h i f t ( ) ,w r i t e l n ( b ) .

?− p .acb

No continuation → Cont = 0

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Applications

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Effect handlers

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Implicit state passing

get/1 & put/1 run state /3

i n c :− get(S) , S1 i s S + 1 , put(S1) .

?− run state ( ( i n c , i n c ) , 0 , S ) .S = 2 .

G. Plotkin & M. Pretnar, Handlers of algebraic effects, ’09

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Implicit state passing

g e t ( S ) :− s h i f t ( g ( S ) ) .

put ( S ) :− s h i f t ( p ( S ) ) .

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g e t ( S ) :− s h i f t ( g ( S ) ) .put ( S ) :− s h i f t ( p ( S ) ) .

r u n s t a t e ( Goal , Sin , Sout ) :−r e s e t ( Goal , Cont , Command ) ,( Cont == 0 →

Sout = S i n; Command = g(S) →

S = Sin ,r u n s t a t e ( Cont , Sin , Sout )

; Command = p(S) →r u n s t a t e ( Cont , S , Sout )

) .

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Definite clause grammars

I Well-known Prolog extension

I Sequential access to elements of implicit list

I Conventionally: program transformation

c/1 phrase/3

Grammar (ab)n

ab ( 0 ) .ab (N) :− c( a ) , c( b ) , ab (M) , N i s M + 1 .

?− p h r a s e ( ab (N) , [ a , b , a , b ] , [ ] ) .N = 2 .

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Definite clause grammars

c (E) :− s h i f t ( c (E ) ) .

p h r a s e ( Goal , L in , Lout ) :−r e s e t ( Goal , Cont , Term ) ,( Cont == 0 →

L i n = Lout; Term = c(E) →

L i n = [ E | Lmid ] ,p h r a s e ( Cont , Lmid , Lout )

) .

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Applications

I Implicit state

I Implicitenvironments

I Logging

I Exceptions:alternate semantics

I Iterators

I Iteratees

I Coroutines

I Search heuristics (Tor)

I . . .

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Composing effect handlers

I Effect handlers easily made compositional

I Propagate unknown operations

Counting ab

ab .ab :− c ( a ) , c ( b ) , inc, ab.

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p h r a s e ( Goal , L in , Lout ) :−r e s e t ( Goal , Cont , Term ) ,( Cont == 0 → . . .; Term = c (E) → . . .;

shift( Term ) ,p h r a s e ( Cont , Lin , Lout )

) .

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Composing effect handlers

run :−r u n s t a t e (

p h r a s e ( ab , [ a , b , a , b ] , [ ] ) ,0 ,N) ,

write (N ) .

?− run .2

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Implementation

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Catch&Throw versus Reset&Shift

Goal :− throw ( Term ) .?− c a t c h ( Goal , B a l l , H a n d l e r ) , . . . .

I unify a copy of Term with Ball

I unwind environment & choice point stacks up to catch/3

I Handler is called before control goes to . . .

Goal :− s h i f t ( Term ) .?− r e s e t ( Goal , Cont , B a l l ) , . . . .

I unify Term with Ball

I leave the stacks intact (protect them)

I unify Cont with copy of the environments up to reset /3

I control goes to . . .

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Four Issues

I up to reset /3

I how to copy (a delimited part of) the environment stack

I how to use this delimited continuation

I fineprint

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Up to reset/3

I catch-throw does this

I so, re-use same principle

I a marker in code or a marker in the environment

I follow the chain of environments, starting at E

protect the stacks: fiddle with the TOS pointer

easy peasy for systems with stack freezing

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Copying a delimited part of the environment stack I

caveat: it’s not a deep copy !

I if continuation is first-class citizen: easy - e.g. BinProlog

I in WAM more complicated:I env contains code pointer, env pointer and termsI copying whole environment as is can be arbitrarily bad in terms

of live environment slotsI so, want selective copy

I determine live env slotsI live variable map (hProlog, YAP, . . . )I decompilation based (SWI-Prolog, SICStus Prolog, . . . )

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Copying a delimited part of the environment stack II

prevE

contP

Env(a/1)

X

YCP

a(X) :− b, c(X,Y), shift(1), d(Y).

The copied environment is a normal Prolog term:

$cont$(abstraction of(CP), 2-[Y] , 2)

A copy of a continuation chain = a list of $cont$/2 terms

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Using a copied continuation

Built-in call tailbody/1 uses $cont$(abstraction of(CP), [Y])

a(X) :− b, c(X,Y), shift(1), d(Y).

Env(a/1)

Y

???

???

P

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Fineprint

I abstraction of(CP)

I non-determinism

I cut

I executing a continuation twice

I errors

I interaction with catch&throw . . .

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Performance

I silly :-)

I catch&throw: linear in depth

I reset & shift : linear in depth and number of variables

goals hProlog SWI-Prologcatch(throw(1), ,true)/reset(shift(1), , ) 2.5/1 2/1depth 10 - no variables 1.4/1 1/3depth 10 - n (live) variables - break even n = 4.5 –

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Summary

Many useful applications: implicit state, implicitenvironments, logging, nonbacktracking exceptions,iterators, iteratees, coroutines, search heuristics, . . .

Program-level definitions

Lightweight implementation in the WAM

I mostly independent

I performance OK

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Do you have . . .

Questions?

Answers?Suggestions?

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