B. Ross Cosc 4f79 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

B. Ross Cosc 4f79 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'

B. Ross Cosc 4f79 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

B. Ross Cosc 4f79 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

B. Ross Cosc 4f79 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

B. Ross Cosc 4f79 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 ***