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            Aachener Informatik-Berichte
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**************** Now available **********************

Filename: 92-20.ps.Z

TITLE    = Implementing Disequality in a Lazy Functional Logic Language

NUMBER   = 92-20
AUTHORS  = H. Kuchen, F.J. Lopez Fraguas, J.J. Moreno Navarro, M. Rodrigues 
           Artalejo
PAGES    = 28
ABSTRACT = In this paper,we investigate an implementation of a lazy functional
           logic language (in particular the language BABEL) which uses 
           disequality constraints for solving equations and building answers. 
           We specify a new operational semantics which combines lazy narrowing 
           with disequality constraints and we define an abstract machine 
           tailored to the execution of BABEL programs according to this 
           semantics. The machine is designed as a quite natural extension of a 
           lazy graph narrowing machine. Disjunctions of disequalities are 
           handled using the backtracking mechanism. 

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Filename: 92-21.ps.Z

TITLE    = Result Directed Computing in a Functional Logic Language

NUMBER   = 92-21
AUTHORS  = H. Kuchen, F.J. Lopez Fraguas
PAGES    = 12
ABSTRACT = In a functional logic language, especially in a constraint functional
           logic language, often the topmost constructor is known which an 
           expression should be evaluated to. E.g.~in a guarded expression 
           $b \rightarrow e$, $b$ should only be evaluated to {\it true}. 
           Evaluating $b$ to {\it false} would only waste resources and risk to 
           run into an infinite computation. Hence, some kind of result directed
           computation can be used to reduce the search space by cutting 
           uninteresting computations. One possibility to achieve this result 
           direction is to integrate it into the abstract machine which is the 
           basis for the runtime system. Here, we show how to achieve result 
           directed computation by means of a program transformation. A runtime 
           system without special facilities for result directed computing can
           be used to run the transformed programs. Thus, runtime overhead for
           result directed computing is avoided. 

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Filename: 92-22.ps.Z

TITLE    = Independent AND-Parallel Narrowing

NUMBER   = 92-22
AUTHORS  = H. Kuchen, J.J. Moreno Navarro, M.V. Hermenegildo
PAGES    = 12
ABSTRACT = We present a parallel graph narrowing machine, which is used to
           implement a functional logic language on a shared memory
           multiprocessor.  It is an extension of an abstract machine for a
           purely functional language.  The result is a programmed graph
           reduction machine which integrates the mechanisms of unification,
           backtracking, and independent and-parallelism.  In the machine, 
           the subexpressions of an expression can run in parallel.
           In the case of backtracking, the structure of an expression is
           used to avoid the reevaluation of subexpressions as far as possible.
           Deterministic computations are detected. Their results are maintained
           and need not be reevaluated after backtracking.
           
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Guido Hogen
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