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prolog

install

sudo apt-get install swi-prolog

basics

  • each line end with .
  • script file with suffix .pl
  • type swipl to launch console
  • if you have temp.pl, type [temp]. to load it
    • after that you can start doing things about the file
  • comment with %
  • :- means \(\leftarrow\)
  • variable
    • capitalized
    • put it in the left
  • terms
    • uncapitalized
    • put it in the right
  • , : AND
  • ; : OR
  • anonymous variable with _
  • equality
    • \= : unequal
  • to make rules symmetric
    • don't mix facts & rules or you might end up in infinite loop
      • love(Alice,Bob) love(X,Y) :- love(Y,X)
    • instead, separate them
      • llove(Alice,Bob) love(X,Y) :- llove(X,Y);llove(Y,X)
  • print
    • write('something')
  • if then
    • prin(X) :- X is 2 -> write('aaa')
    • use ; to else

list

[X|Y]=[a,b,c,d,e]
%will return
%X=a
%Y=[b,c,d,e]
anonymous variable with _ 
[_,X|_]=[a,b,c,d,e]
%will return
%X=b

examples

in file temp.pl 

big(bear).             % Tell Prolog that a bear is big
big(elephant).         % Tell Prolog an elephant is big
small(cat).            % Tell Prolog a cat is small

brown(bear).           % Tell Prolog a bear is brown
black(cat).            % Tell Prolog a cat is black
gray(elephant).        % Tell Prolog an elephant is gray

dark(X) :- black(X).   % Tell Prolog that anything is dark IF it is black
dark(X) :- brown(X).   % Tell Prolog that anything is dark IF it is brown
in console (line by line) 1. [temp]. 2. dark(X). - will return - X = cat; X = bear. - need to press ; to output next result

resources

  • blog
    • https://blog.bhavul.com/prolog-mind-blown/
  • nice comprehensive slides
    • http://www.let.rug.nl/bos/lpn//lpnpage.php?pageid=teaching
      • list view http://www.let.rug.nl/bos/lpn/slides/official/
  • 99 problems & solutions
    • https://www.ic.unicamp.br/~meidanis/courses/mc336/2009s2/prolog/problemas/