PROLOG

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PROLOG

• PRAGMATICS
• Chapter 7

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• There are a wide variety of topics covered in
  this chapter.
• Well address only the most important.
• Read the entire chapter to be aware of these
  capabilities.

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Disjunction ()

• Works exactly as it would in logic, Prolog
  interprets "" to mean "or
• When a "" is placed between two lines
  (expressions), Prolog tries to solve the first
  expression. If it cannot, then it attempts to
  solve the second expression.
• Not commonly used.

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Example Disjunction - Query

• language(postscipt).
• food(burrito).
• language(postscript)food(wood).
• yes.

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Example Disjunction - Predicate

• parent(Parent,Child)-
• mother(Parent,Child)
• father(Parent,Child).
• The traditional style is to write two versions of
  the predicate
• parent(Parent,Child)-
• mother(Parent,Child).
• parent(Parent,Child)-
• father(Parent,Child).
• Each version is treated as OR

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If-then and if-then-else (-gt)

• Two forms
• Condition -gt Truegoal
• Condition -gt Truegoal Falsegoal
• For the first form, if the condition is true,
  execute Truegoal.
• For the second form, if the condition is true,
  execute Truegoal if the condition is false,
  execute Falsegoal.
• Can accomplish the same logic by defining two
  predicates, and adding nots and ands.

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Cut (!)

• Cut tells the program to discard the choices it
  has made up to that point.
• Recall for backtracking, Prolog places markers at
  all decision points which are then used to
  backtrack. When cut is encountered, all markers
  are deleted.
• Reduces the search space, often quite
  dramatically. Making a program much more
  efficient in terms of search time and memory
  requirements.
• Sometimes required for a program to work the way
  you want it to.

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Cut Example

• old(X)- age(X,A),A gt 65 .
• age(jo,30).
• age(bill,76).
• age(zeke,98).
• The query old(Names). causes Prolog to search
  through the data base to find all the old people.

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Cut Example (continued)

• old(jo).
• Prolog will respond "no", but it will also
  attempt to backtrack, searching the knowledge
  base for all of jos ages.
• However, as the programmer, you know that each
  individual has only one age and backtracking is
  useless.
• To control backtracking, alter the predicate

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Cut Example (continued)

• old2(X)- age(X,A), !, A gt 65 .
• To execute the query, Prolog will instantiate X
  and then remove the markers made previous to !
• This has the effect of forcing Prolog to consider
  only the choices in effect when it reached the
  !.
• old2(jo). - no longer backtracks
• old2(Names). - cannot find the names of the old
  people, because it is unable to backtrack.

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Cut Cautions

• Be careful when using cut!
• It may work fine when you provide values for
  certain arguments, but how the predicate will
  behave when you provide values for different
  arguments (or no values at all) can be tricky.
• Test predicates that use ! thoroughly and
  document the limits on the use of the predicate.

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Application of Cut, Disjunction and Condition

• Cut is used in applications. In some cases it is
  the only way to implement the needed behavior.
• I recommend against the use of disjunction and
  condition.

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Fail

• Means just what it says - Fail (false).
• Often used with cut.

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Fail Example

• Define a predicate "canFly(X)" which only
  succeeds when X can fly.
• If X is a bird, but not an ostrich or penguin it
  should succeed.

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Fail Example (continued)

• is_a(bird,ostrich).
• is_a(bird,penguin).
• is_a(bird,parakeet).
• is_a(ostrich,ted).
• is_a(penguin,bill).
• is_a(parakeet,kim).
• canFly(X)- is_a(ostrich,X),!,fail.
• canFly(X)- is_a(penguin,X),!,fail.
• canFly(X)- is_a(Y,X),is_a(bird,Y).

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Fail Example (continued)

• canFly(bill).
• canFly(ted).
• canFly(kim).
• canFly(X).
• What happens if the !fail combination is not
  included
• The query succeeds via the third version of the
  predicate.

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Built in Predicates

• Negation (not)
• Assert
• Retract
• ..
• Arg

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Negation (not)

• not (Goal)
• not Goal
• Reverses the truth value of the goal. (Think in
  terms of logic!)

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Assert

• asserta(clause) - adds clause the first clause
  with this name in the database (i.e., puts it at
  the top).
• assertz(clause) - make this the last clause with
  this name in the database (i.e., put it at the
  bottom)
• The parameter can be almost anything, another
  rule, a fact, etc.
• VERY useful. Provides the ability to alter the
  database during execution.

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Retract

• retract(clause) - removes the first clause whose
  head matches from the database.
• Frequently used in conjunction with assert to
  save and recall values from the database.

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Abolish

• Removes all versions of the predicate from the
  database.
• Example
• abolish(clause)

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..

• Term .. List
• Converts a list (on the right) to a term (on the
  left)
• To allow conversion between lists and facts.
• Sets "List" to a list whose first element is the
  predicate of X and whose other terms are the
  arguments of X.

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.. Examples

• loves(quince,lox).. Y?
• Y loves,quince,lox
• X..a,b,c,d,e
• Xa(b,c,d,e)

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arg(N,Structure,Argument)

• Unifies Argument with the Nth argument of
  Structure
• N must be a positive integer, and Structure
  should be a compound term with arity greater than
  or equal to N. Must specify values for N and
  Structure.

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Arg Examples

• arg(2, related(john,mother(jane) ),X).
• X mother(jane)
• arg(1, a(bc),X).
• X a

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Checking the Types of Terms

• Refer to text page 191
• atom(Term) Term is an atom.
• atomic(Term) Term is an atom or number.
• float(Term) Term is a floating point number.
  Only examines the internal representation.
• integer(Term) Term is an integer.
• integer(2.0) is true
• integer range is -2 27 to 227-1

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Types of Terms

• number(Term) Term is integer or floating point.
• var(Term) Term is an unbound variable.
• nonvar(Term) Term is an instantiated variable.
• Term is instantiated if it has been assigned a
  value, or if Prolog has assigned a value during
  processing).

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Input and Output

• write
• read
• see
• tell
• ratom

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write and writeq

• write(X) were X is the term to be written to the
  output stream.
• writeq(X) places quotes around X
• Other handy terms
• nl - tells Prolog to move to a new line.
• tab(N) - tells Prolog to output N spaces (ie.
  tab over)

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read

• read(Term) reads the next Term.
• A term ends with a period or question mark.
• get(C) gets the next printable Character in the
  input stream.
• Geto(C) gets the next Character.

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See

• see, seeing, and seen are ways to define,
  determine and change the current input stream.
  By default the I/O stream is the screen.
• see(File) tells Prolog to set the input stream to
  File.
• seeing(File) has Prolog return the name of the
  current input stream.
• seen closes the current input stream.

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Tell

• tell, telling, and told are ways to define,
  determine, and close the current output stream.
• tell(File) tells Prolog to direct output to File.
• telling(File) has Prolog return the name of the
  current output stream.
• told closes the current output stream (close also
  works).

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Ratom

• Ratom(X) reads an atom (input token, single
  character).
• Does not read .