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Inference mechanisms

Inference mechanisms. • Backward chaining - goal-driven reasoning which gathers data as needed - Prolog's default mechanism - good for identification problems • Forward chaining - data-driven reasoning which must be initialized with all data values

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Inference mechanisms

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  1. Inference mechanisms • Backward chaining - goal-driven reasoning which gathers data as needed - Prolog's default mechanism - good for identification problems • Forward chaining - data-driven reasoning which must be initialized with all data values - system refines problem state towards solution state - good for configuration problems

  2. Misc Prolog notes 1. With Sicstus prolog, need to do either a "nl" or a "ttyflush" command in order to see output on screen before a read - these commands flush the output buffer (similar things happen in C) 2. With Prolog's "read", a capitalized word will be interpreted as a variable. eg. read(X) --> enter Yes means X is unified with variable Yes enter 'Yes' unifies X with constant 'Yes' enter yes unifies X with constant 'yes'

  3. Separation of KB and shell • Important to separate declarative KB code from procedural shell utilities - knowledge base can then be used by other inference schemes - shell can be altered and made as flexible as one needs, w/o touching KB - Basically, put each into distinct files. • In Merritt "bird" system, one line in KB is: top_goal(X) :- bird(X). • Then, shell always executes "top_goal(X)". --> this gives a hook between KB and shell

  4. Bowen Toy system • from “Prolog and Expert Systems”, K.A. Bowen (McGraw Hill) • similar in spirit to Bird • he handles some things differently eg. to keep track of similar input words: synonym(pain, severe_pain). synonym(pain, numb_pain). ... then you can check synonym for user input X by doing: ?- synonym(S, X). • he also uses setof(V, synonym(pain,V), Vlist) --> returns Vlist = [severe_pain, numb_pain, ...] - setof(Template, Goal, List): successively solves Goal, and saves variables shared between Goal and Template in List, using Template - sorts List, and removes duplicates - Template is a term: a variables, or even a structure eg. p(X,Y) • bagof: similar, except that it keeps all solutions in the order discovered, and doesn't sort nor remove duplicates

  5. Friendly User Input • Prolog's Input-Output is too unfriendly and unforgiving • should develop a library of user-friendy I/O • expert system interface: - users may have little or no computer background - users may have little or no background in domain area - recover from errors (typing, misunderstandings, ...) - avoid unnecessary dialog - remember input - use menus - perhaps remember input from different sessions - keep a history database - provide various levels of explanation and help

  6. Summary • The knowledge base and shell utilities are separate. • The knowledge base is declarative. It can be processed by any kind of inference system. It is easily modified. • The system prompts user for facts. It remembers input. • Menu input is used. • Note that, although shell utilities are procedural, it is possible to write declarative utilities. This is an ideal, but they might be inefficient. • possible enhancements: - Better text messages for queries - more error checking - numeric input with menus (see handout) - *** explanation ***

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