Introduction to PrologIntroduction to Prolog

Terms & MatchingTerms & Matching

MathMath

Atoms and TermsAtoms and Terms

mark, alex, di, bob are mark, alex, di, bob are atomsatoms– Not variablesNot variables– Not stringsNot strings– Just Just things – simple things – simple thingsthings

There are more kinds of atomsThere are more kinds of atoms There are also There are also terms terms & & listslists

– Complex Complex or or compound compound thingsthings

AtomsAtoms

Identifiers starting with lower-case letterIdentifiers starting with lower-case letter– markmark, , didi, , bobbob

Numbers: integers & floating pointNumbers: integers & floating point– 77, , 12081208, , -4-4, , 3.14163.1416, , 6.02e236.02e23

Sequences of special charactersSequences of special characters– ==, , :-:-, , ->->, , *->*->, , <=*=><=*=>

Single-quoted stringsSingle-quoted strings– ‘‘Tom’Tom’, , ‘23-skidoo’‘23-skidoo’, , ‘I am not a number!’‘I am not a number!’, , ‘3’‘3’

TermsTerms

Term consists of functor and argumentsTerm consists of functor and arguments– Just like a factJust like a fact

Functor says what Functor says what kindkind of a thing it is of a thing it is– Arguments differentiate it from othersArguments differentiate it from others

point(2, 5)point(2, 5)

point(7, –13)point(7, –13)

date(2002, jan, 17)date(2002, jan, 17)– Terms can be arguments in facts, rules & Terms can be arguments in facts, rules & termsterms

Two different points on the plane

Today’s date

Using DatesUsing Dates

%% date(Year, Month, Day)%% date(Year, Month, Day)

%% Gregorian calendar dateGregorian calendar date

% % Year is a numberYear is a number

% % Month is a TLA for a monthMonth is a TLA for a month

% % Day is a number in the range 1..31Day is a number in the range 1..31

%% born(Person, DateOfBirth)%% born(Person, DateOfBirth)

born(alex, date(1994, apr, 24)).born(alex, date(1994, apr, 24)).

born(zachary, date(2001, dec, 4)).born(zachary, date(2001, dec, 4)).

ExerciseExercise

Write born/2 for the members of your Write born/2 for the members of your familyfamily

born(alex, date(1994, apr, 24)).born(alex, date(1994, apr, 24)).

Matching TermsMatching Terms

Variables match anything: atom, term, …Variables match anything: atom, term, …?- ?- born(alex, Date).born(alex, Date).Date = date(1994, apr, 24)Date = date(1994, apr, 24)YesYes Can put variables Can put variables inin terms terms?- ?- born(Who, date(1994, _Month, _Day)).born(Who, date(1994, _Month, _Day)).Who = alexWho = alexYesYes

What’s Alex’s date of birth?

Who was born in 1994?

Rules with TermsRules with Terms

Can use terms in rulesCan use terms in rulesborn_in_year(Who, Year) :-born_in_year(Who, Year) :-

born(Who, date(Year, _Month, _Day)).born(Who, date(Year, _Month, _Day)).born_in_month(Who, Month) :-born_in_month(Who, Month) :-

born(Who, date(_Year, Month, _Day)).born(Who, date(_Year, Month, _Day)). Everything works just like you’d expectEverything works just like you’d expect?- ?- born_in_month(zachary, M).born_in_month(zachary, M).M = decM = dec

Non-Printing Variables in RulesNon-Printing Variables in Rules

Suppose we’d written:Suppose we’d written:born_in_year(Who, Year) :-born_in_year(Who, Year) :-

born(Who, date(Year, Month, Day)).born(Who, date(Year, Month, Day)). Code works, but Prolog complainsCode works, but Prolog complains

– Warning: Singleton variables: [Month, Day]Warning: Singleton variables: [Month, Day] Use variables to make two arguments sameUse variables to make two arguments same

– so wonders why those variables only used onceso wonders why those variables only used once

Single Single UseUse Variables Variables

Variables used only once may be a mistakeVariables used only once may be a mistake– typed one name wrongtyped one name wrong

Using _Var lets Prolog know that you know Using _Var lets Prolog know that you know that it’s only used once – and that’s OKthat it’s only used once – and that’s OK

Can also use Can also use anonymousanonymous variable, _ variable, _– ““anonymous” variable = has no nameanonymous” variable = has no name– use it multiple times – each a different variableuse it multiple times – each a different variable

Anonymous VariablesAnonymous Variables

Used when you *really* don’t care what Used when you *really* don’t care what value is filled invalue is filled in– don’t want it printed outdon’t want it printed out– don’t need it to be the same as any otherdon’t need it to be the same as any other

Each use of _ is a different variableEach use of _ is a different variableborn_in_year(Who, Year) :-born_in_year(Who, Year) :-

born(Who, date(Year, _, _)).born(Who, date(Year, _, _)).

Anonymous VariablesAnonymous Variables

Can use at the top level, tooCan use at the top level, too– doesn’t print the value (starts with _)doesn’t print the value (starts with _)– a different variable each timea different variable each time– (note – they (note – they maymay get the same value) get the same value)

?- ?- parent(P, _), parent(_, C).parent(P, _), parent(_, C).

P = markP = markC = alexC = alexyesyes

Q: Are there P and C such that -- P is someone’s parent, and C is someone’s child?A: yes, mark is someone’s parent and alex is

someone’s child

On Using Anonymous VariablesOn Using Anonymous Variables

In general, use named variablesIn general, use named variables Start with _ if used only onceStart with _ if used only once Use anonymous variable only if there are a Use anonymous variable only if there are a

lot of single-use variables & no good, short lot of single-use variables & no good, short names to use for themnames to use for them

born_in_year(Who, Year) :-born_in_year(Who, Year) :-born(Who, date(Year, _Month, _Day)).born(Who, date(Year, _Month, _Day)).

ExerciseExercise

Write a rule for born_in_same_year/2, which Write a rule for born_in_same_year/2, which says that the two people mentioned in the says that the two people mentioned in the arguments were born in the same year.arguments were born in the same year.

%% born_in_same_year(Person_1, Person_2)%% born_in_same_year(Person_1, Person_2)% % Person_1 & Person_2 born in same yearPerson_1 & Person_2 born in same year%% Note: perhaps on different days!Note: perhaps on different days!

born_in_same_year(Person_1, Person_2) :-born_in_same_year(Person_1, Person_2) :-

……..

Terms in TermsTerms in Terms

Line segment defined by two pointsLine segment defined by two pointsseg( point(0, 0), point(4, 3) )seg( point(0, 0), point(4, 3) )seg( point(1, 1), point(3, 4) )seg( point(1, 1), point(3, 4) ) Triangle defined by three pointsTriangle defined by three pointstriangle( point(0, 0), point(4, 3), point(3, 4) )triangle( point(0, 0), point(4, 3), point(3, 4) ) Two lines determine an angleTwo lines determine an angleangle( seg( point(0, 0), point(4, 3) ), angle( seg( point(0, 0), point(4, 3) ),

seg( point(1, 1), point(3, 4) ) ) seg( point(1, 1), point(3, 4) ) )

Terms in Facts and RulesTerms in Facts and Rules

A line segment is A line segment is verticalvertical if one point is if one point is directly above the otherdirectly above the other– The two points have the same X coordinateThe two points have the same X coordinate

vertical( seg( point(X, _Y1), point(X, _Y2) ) ).vertical( seg( point(X, _Y1), point(X, _Y2) ) ). And And that’sthat’s a fact a fact

– Note: vertical/1 – the only arg. is a line seg.Note: vertical/1 – the only arg. is a line seg.– Has only one clause in its definitionHas only one clause in its definition– Variables appear Variables appear inside inside the termthe term

Asking About Line SegmentsAsking About Line Segments

Is the line segment from (3,4) to (3,7) Is the line segment from (3,4) to (3,7) vertical?vertical?

?- ?- vertical( seg( point(3, 4), point(3, 7) ) ).vertical( seg( point(3, 4), point(3, 7) ) ).yesyes How about from (0,0) to (3,4)?How about from (0,0) to (3,4)??- ?- vertical( seg( point(0, 0), point(3, 4) ) ).vertical( seg( point(0, 0), point(3, 4) ) ).nono X can’t be both 0 and 3

they have the same X coordinate

Asking About Line SegmentsAsking About Line Segments

vertical( seg( point(X, Y1), point(X, Y2) ) ).vertical( seg( point(X, Y1), point(X, Y2) ) ).

?- ?- vertical( seg( point(3, 4), point(3, 7) ) ).vertical( seg( point(3, 4), point(3, 7) ) ).

X = 3, Y1 = 4, X = 3, Y2 = 7X = 3, Y1 = 4, X = 3, Y2 = 7yesyes

vertical( seg( point(X, Y1), point(X, Y2) ) ).vertical( seg( point(X, Y1), point(X, Y2) ) ).

?- ?- vertical( seg( point(0, 0), point(3, 4) ) ).vertical( seg( point(0, 0), point(3, 4) ) ).

X = 0, Y1 = 0, X = 3 (X = 0, Y1 = 0, X = 3 (failsfails))nono

Generating PointsGenerating Points

Is there line segment between (3,4) and a Is there line segment between (3,4) and a point on the x-axis that is vertical?point on the x-axis that is vertical?– On the x-axis means has y-coordinate zeroOn the x-axis means has y-coordinate zero

?- ?- vertical( seg( point(3, 4), point(X, 0) ) ).vertical( seg( point(3, 4), point(X, 0) ) ).X = 3X = 3yesyes Yes there is; it has x-coordinate 3Yes there is; it has x-coordinate 3

Asking About Line SegmentsAsking About Line Segments

vertical( seg( point(Xvertical( seg( point(X00, Y1), point(X, Y1), point(X00, Y2) ) )., Y2) ) ).

?- ?- vertical( seg( point(3, 4), point(X, 0) ) ).vertical( seg( point(3, 4), point(X, 0) ) ).

XX00 = 3, Y1 = 4, X = 3, Y1 = 4, X00 = X, Y2 = 0 = X, Y2 = 0

X = 3X = 3yesyes

Variables in VariablesVariables in Variables

Same as above – different way to askSame as above – different way to ask?- ?- P = point(_, 0), vertical( seg( point(3, 4), P ) ).P = point(_, 0), vertical( seg( point(3, 4), P ) ).P = point(3, 0)P = point(3, 0)yesyes Equals sign just gives a value to a variableEquals sign just gives a value to a variable Value of _ not given explicitly (anonymous)Value of _ not given explicitly (anonymous) Complete Complete value of P printed (no underscore)value of P printed (no underscore)

– Value of _ isValue of _ is part part of P’s valueof P’s value

Note Order IndependenceNote Order Independence

Can put P = point(_,0) before or afterCan put P = point(_,0) before or after?- ?- vertical( seg( point(3, 4), P ) ), P = point(_, 0).vertical( seg( point(3, 4), P ) ), P = point(_, 0).P = point(3,0)P = point(3,0)yesyes Also note symmetry of =Also note symmetry of =?- ?- vertical( seg( point(3, 4), P ) ), point(_, 0) = P.vertical( seg( point(3, 4), P ) ), point(_, 0) = P.P = point(3,0)P = point(3,0)yesyes

Equals is Equals is NotNot Assignment Assignment

Equals sign does not Equals sign does not change change the valuethe value– Just Just matchesmatches one side with the other one side with the other

?- ?- P = mark, P = di.P = mark, P = di.nono P is mark – it P is mark – it has has to to satisfy first =to to satisfy first =

– So it can’t So it can’t alsoalso be di be di

Equals Equals IsIs Matching Matching

Try to fill in all the variables so that the two Try to fill in all the variables so that the two sides are sides are exactlyexactly the same the same

Is there a point on both the x-axis and the y-Is there a point on both the x-axis and the y-axis?axis?

?- ?- point(X, 0) = point(0, Y).point(X, 0) = point(0, Y).X = 0X = 0Y = 0Y = 0yesyes

point(X,0) is a point somewhere on the x-axispoint(0,Y) is a point somewhere on the y-axisWe want them to be the same point, so use =

MatchingMatching

Get all the pieces to be the sameGet all the pieces to be the same– variable matches anythingvariable matches anything– atom matches same atomatom matches same atom– term matches term with same functor and same term matches term with same functor and same

argumentsarguments

?- ?- bob(1, 2, X) = bob(X, 2, Y).bob(1, 2, X) = bob(X, 2, Y).

X = 1X = 1

Y = 1Y = 1

bob matches bob1 matches X2 matches 2X (1) matches Y

ExerciseExercise

Do these match? What values do Vars get?Do these match? What values do Vars get?– fred and Ffred and F– fred and wilmafred and wilma– Thelma and louise(1,2,3)Thelma and louise(1,2,3)– tom(a,b,c) and tommy(a,b,c)tom(a,b,c) and tommy(a,b,c)– seg(pt(X,1), pt(2,X)) and seg(Z, pt(U,3))seg(pt(X,1), pt(2,X)) and seg(Z, pt(U,3))– a(X,Y,Y,Z) and a(1,2,Z,3)a(X,Y,Y,Z) and a(1,2,Z,3)

Making It MatchMaking It Match

seg(pt(X,1), pt(2,X))seg(pt(X,1), pt(2,X)) seg( Z, pt(U,3))seg( Z, pt(U,3))

Z=pt(X,1)Z=pt(X,1)

U=2U=2

X=3X=3

Z=pt(3,1)Z=pt(3,1)

Failing to MatchFailing to Match

a(X,Y,Y,Z)a(X,Y,Y,Z) a(1,2,Z,3)a(1,2,Z,3)

X=1X=1

Y=2Y=2

Y=ZY=Z

Z=2Z=2

Z=3 Z=3 impossible: Z=2impossible: Z=2

ExerciseExercise

Don’t usually use =/2Don’t usually use =/2– just let Prolog give variables a valuejust let Prolog give variables a value?- ?- on_x_axis(P), vertical( seg( point(3,4), P ) ).on_x_axis(P), vertical( seg( point(3,4), P ) ).P = point(3, 0)P = point(3, 0)YesYes

A point is on the A point is on the xx axis if its axis if its yy coordinate is coordinate is zero. Write a zero. Write a factfact to express that. to express that.on_x_axis( … ).on_x_axis( … ).

Equals is Equals is NotNot Math Math

Beginners often do this:Beginners often do this:?- ?- X = 3 * 17 – 12 * 5.X = 3 * 17 – 12 * 5.X = 3*17 – 12*5X = 3*17 – 12*5yesyes It didn’t do the math!It didn’t do the math! Equality is Equality is matchingmatching

– Math is not (just) matchingMath is not (just) matching

So What About MathSo What About Math

Use the is/2 predicate (infix syntax, like =)Use the is/2 predicate (infix syntax, like =)?- ?- X is 3 * 17 – 12 * 5.X is 3 * 17 – 12 * 5.X = –9X = –9yesyes Can use all usual math operations and Can use all usual math operations and

functionsfunctions– Multiply with *, divide /, power ^ or **Multiply with *, divide /, power ^ or **

Calculating DistanceCalculating Distance

Calculate distance between pointsCalculate distance between pointsdistance(point(X1,Y1), point(X2,Y2), D) :-distance(point(X1,Y1), point(X2,Y2), D) :-

D is sqrt((X1–X2)^2 + (Y1–Y2)^2).D is sqrt((X1–X2)^2 + (Y1–Y2)^2). Will calculate a distanceWill calculate a distance?- ?- distance(point(0,0), point(3,4), D).distance(point(0,0), point(3,4), D).D = 5D = 5yesyes

(Not) Calculating Points(Not) Calculating Points

Will Will not not calculate a point to fit a distancecalculate a point to fit a distance

?- ?- distance(point(0,0), point(3,Y), 5).distance(point(0,0), point(3,Y), 5).ERROR: Arguments are not sufficiently instantiatedERROR: Arguments are not sufficiently instantiated^ Exception: (7) …^ Exception: (7) …

Type an Type an nn on the exception line on the exception line– Prolog says “no debug” and returns to promptProlog says “no debug” and returns to prompt– Couldn’t calculate D because didn’t know YCouldn’t calculate D because didn’t know Y

Note: SWI-Prolog throws exceptionsNote: SWI-Prolog throws exceptions

““Extra-Logical” PredicatesExtra-Logical” Predicates

Some predicates don’t commuteSome predicates don’t commute– Y = 5, X is Y + 1 Y = 5, X is Y + 1 finefine– X is Y + 1, Y = 5X is Y + 1, Y = 5 no goodno good

Since logical AND is commutative, Since logical AND is commutative, Prolog’s comma is not Prolog’s comma is not quitequite the same the same– Prolog has a Prolog has a flow of controlflow of control– It is a programming language, after allIt is a programming language, after all

Aside: ConstraintsAside: Constraints

Some Prologs allow under-specified mathSome Prologs allow under-specified math– and other under-specified stuff as welland other under-specified stuff as well

““Freeze” a computation until it’s Freeze” a computation until it’s instantiated “enough”instantiated “enough”– {X is Y + 1}, Y = 5{X is Y + 1}, Y = 5 fine in BNR Prolog fine in BNR Prolog– Waits until it knows Y before it calculates XWaits until it knows Y before it calculates X– AKA “constraint” on XAKA “constraint” on X

Aside: Logical MathAside: Logical Math

Some Prologs can do logical mathSome Prologs can do logical math– give two arguments…give two arguments…– ……it fills in the thirdit fills in the third

?- ?- sum( 5, X, 12 ), product( X, Y, 63 ).sum( 5, X, 12 ), product( X, Y, 63 ).

X = 7X = 7

Y = 9Y = 9

YesYes

Atoms, Terms & ListsAtoms, Terms & Lists

AtomAtom– indivisible itemindivisible item

TermTerm– compound itemcompound item

ListList– sequence of itemssequence of items– (or of items & lists)(or of items & lists)– lots of useful built-in predicates for theselots of useful built-in predicates for these

ListsLists

List = sequence of valuesList = sequence of values– [bob, brian, cathy, mark, david, loretta][bob, brian, cathy, mark, david, loretta]– [1, 2, 3, 4, 5][1, 2, 3, 4, 5]– [birds(4, calling), hens(3, french), doves(2, turtle), [birds(4, calling), hens(3, french), doves(2, turtle),

partridge(1, in(tree(1,pear)))]partridge(1, in(tree(1,pear)))]– [X, Y, Z, Z, Y][X, Y, Z, Z, Y]– [1, brian, doves(2, turtle), Z][1, brian, doves(2, turtle), Z]

Representations of ListsRepresentations of Lists

Written in [square] bracketsWritten in [square] brackets– Note: (parentheses), [brackets], {braces}Note: (parentheses), [brackets], {braces}– mean different things in computer languagesmean different things in computer languages

Elements, separated, by, commasElements, separated, by, commas Each element can be Each element can be anythinganything

– atom, term, variable, or listatom, term, variable, or list Simplest list is the empty list: []Simplest list is the empty list: []

Facts with ListsFacts with Lists

List can be an argument of a factList can be an argument of a fact– or of any termor of any term

%% family(Husband, Wife, Children)%% family(Husband, Wife, Children)family(mark, di, [alex, zachary]).family(mark, di, [alex, zachary]).

%% isa_polygon( P )%% isa_polygon( P )isa_polygon( poly([P1, P2, P3 | PMore]) ) :-isa_polygon( poly([P1, P2, P3 | PMore]) ) :-

are_points([P1, P2, P3 | PMore]).are_points([P1, P2, P3 | PMore]).

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More on listsMore on lists Sections 3.1 & 3.2Sections 3.1 & 3.2