Input and output modes are determined the initial goal

Metamorphosis 97

[Item1|List1]«List1occurs in the guard of two clauses. Composing guard con-straints into decision trees can eliminate redundant tests:

//partition(List,Pivot,Lesser,Greater)
part(List,Pivot,Lesser,Greater)
:- []=List
| Lesser=[] | Greater=[]
+ :- [Item|List1]=List,
{Pivot=<Item
| Greater=[Item|Greater1],
+ otherwise
| Lesser=[Item|Lesser1], part(List,Pivot,Lesser1,Greater) }.

using otherwise (or else). The otherwise clause is only tried if all others are inappli-cable.

Chapter 3

Van Emden and de Luceana [1979] embed the Kahn and MacQueen model of stream processing in logic programming. This gave a process interpreta-tion of goals in contrast to the previous procedural interpretation. Goals in a query or body of a clause are grouped into sequential and parallel conjunc-tions. Variables shared between parallel conjunctions are regarded as com-munication channels. Parallel processes suspend if shared variables are not instantiated to lists. Input and output modes are determined by the initial goal. The examples given are deterministic. The authors have great diffi-culty in getting their paper accepted for publication. It is not published until three years later as an invited contribution to a book [Clark and Tarnlund, 1982].

1980

1981	Conery and Kibler [1981] identified four prospects for parallelism in logic programs: and-parallelism, or-parallelism, stream-parallelism and search-parallelism. In search-parallelism, matching clauses are searched for in par-allel.