AI1 Prolog exercise 2                            20.11.84				

  Check that you have a copy of emnotes (notes on the editor em).
  Switch the terminal on; call host (2972); log on.

  Look at your files - make sure you still have a copy of the file
  called 'family'. Copy it from me if you haven't (ecmi04.famtree).

  Run prolog - use ai1prolog or p

    Command: ai1prolog

  Consult the file 'family'

    ?- [family].

  Try matching a few goals to remind yourself how it works.

    ?- parent(Who,cecil).

    ?- daughter(tom,D).

    ?- son(mary,P).

    ?- son(P,mary).

  Quit from prolog

    ?- quit.

  Last week we added extra clauses to our family tree program
  by consulting a special file - the user file. Each clause
  was read in from the terminal, instead of from a stored file
  in your file area. All the clauses added were lost when you
  'quitted' prolog. If we put the clauses in a file in your
  file area (instead of in the user file) we could consult it
  (as we consulted 'family') and use the clauses, and save them
  to be used again. We can also change them easily.

  We are going to add a grandparent clause and more family.
  The editor 'em' allows us to add/delete/change/move text
  in a file. Type the following:

    Command: em family

  Use your emnotes and try a few commands to the editor.

    >1p
    >5p
    >1,10p
    >10
  The last line altered or printed becomes 'the current line',
  indicated by . (a single full stop). The last line in the 
  file is indicated by $. Try
    >p999
    >1,$p
  
  Try adding some more people to the database.

    >1p
    >a
    father(alan,tom).
    father(alan,jane).
    father(alan,fred).
    mother(zoe,jim).
    mother(zoe,sue).
    .
    >

  Remember a single . on its own indicates that you have finished
  adding text. Look out for the > prompt to continue with edit
  commands.

  We could also add the grandparent rule. After the male/female
  clauses might be a good place to put it, before the parent
  clauses.

    >/parent/

  Find the first line with 'parent' in it.
  Insert the new rule before it.

    >i
    grandparent(Gp,Gch):-
        parent(Gp,Pa),
        parent(Pa,Gch).
    .
    > 1,$p

  The whole database will be listed. We need to 'write it back'
  to the file (ie update the file to include what we have changed
  in this session).

    >w

  Get back to emas command level (use q not quit)

    >q

  Get into prolog. Consult your new version of 'family'.
  List it. Try it. If you spot any errors you will need to
  edit the file again to alter it.

    Command: list family
    Command: p

    ?- [family].
    ?- listing.

  There is a way to edit the file without having to go out of
  prolog and back in again. You can give emas commands from
  inside prolog.

    ?- emas(em,family).

  will have the same effect as 

    Command: em family

  except that when you finish editing you will be back in
  Prolog again automatically (if that is where you went into
  the editor from).

    >w
    >q

    ?- [-family].

  You have to consult the file again BUT if you just do this
  as before [family] you will  get two copies of some clauses
  in your database. To prevent this you reconsult the file,
  and all clauses in the new (reconsulted) file replace any in
  the old file with the same name. To indicate to the prolog
  interpreter that you wish to reconsult you put a - in front
  of the filename.

  Try adding brother/sister and aunt/uncle clauses:
    write them out on paper first
    get into the editor
    add/change the text
    get back to prolog
    reconsult the file
    list it
    test it
    look any changes needed
    start again!


  If you get this far, and are quite happy, then try something 
  harder!

  How would you add facts and rules (ie clauses) to the database
  to give information about people's ages, and to be able to
  answer questions like 'Is fred older than Jim?' or 'How old
  is Sue?' ?

  Some hints:

    You could have a clause called age with two arguments
    (persons_name,persons_age) eg age(fred,30). age(mary,72).

    You can compare ages using > which means 'is greater than'
    ie X > Y is true (the goal succeeds) if whatever matches to
    X is greater that whatever matches to Y
    eg  if X becomes instantiated to 72 and y to 30 then
    the goal X > Y would succeed.
      try ?- age(cecil,X),age(mary,Y), Y>X.
    [you have to add some ages to the database first]

    There might be a rule 
              is_older(Oldperson,Youngperson)
    which is true when the Oldperson is older than the Youngperson.
    The whole rule, if we were comparing mary and cecil's ages,
    would be
          mary is older than cecil if
               we know mary's age and
               we now cecil's age and
               mary's age is greater than cecil's age.

    We could different rules for cases where we don't know
    everyone's age, but we know that if fred is the parent
    of cecil then fred must be older than cecil.
    How about this
            mary is older than cecil if
                  mary is older than someone and
                  that someone is older than cecil.

    [we are getting into more hairy ground here with clauses that
     have subgoals with the same name as themselves - if a clause
     calls a clause of the same name (ie it has itself as a subgoal)
     we say that it is recursive - more of this later in the course].