9.6 Practical Session

In this practical session, we want to introduce some built-in predicates for printing terms onto the screen. The first predicate we want to look at is display/1, which takes a term and prints it onto the screen.

?- display(loves(vincent,mia)).
loves(vincent, mia)
 
Yes
?- display('jules eats a big kahuna burger').
jules eats a big kahuna burger
 
Yes

More strictly speaking, display prints Prolog's internal representation of terms.

?- display(2+3+4).
+(+(2, 3), 4)
 
Yes

In fact, this property of display makes it a very useful tool for learning how operators work in Prolog. So, before going on to learn more about how to write things onto the screen, try the following queries. Make sure you understand why Prolog answers the way it does.

?- display([a,b,c]).
?- display(3 is 4 + 5 / 3).
?- display(3 is (4 + 5) / 3).
?- display((a:-b,c,d)).
?- display(a:-b,c,d).

So, display is nice to look at the internal representation of terms in operator notation, but usually we would probably prefer to print the user friendly notation instead. Especially when printing lists, it would be much nicer to get [a,b,c], instead of .(a.(b.(c,[]))). This is what the built-in predicate write/1 does. It takes a term and prints it to the screen in the user friendly notation.

?- write(2+3+4).
2+3+4
 
Yes
?- write(+(2,3)).
2+3
 
Yes
?- write([a,b,c]).
[a, b, c]
 
Yes
?- write(.(a,.(b,[]))).
[a, b]
 
Yes

And here is what happens, when the term that is to be written contains variables.

?- write(X).
_G204
 
X = _G204
yes
?- X = a, write(X).
a
 
X = a
Yes

The following example shows what happens when you put two write commands one after the other.

?- write(a),write(b).
ab
 
Yes

Prolog just executes one after the other without putting any space in between the output of the different write commands. Of course, you can tell Prolog to print spaces by telling it to write the term ' '.

?- write(a),write(' '),write(b).
a b
 
Yes

And if you want more than one space, for example five blanks, you can tell Prolog to write '     '.

?- write(a),write('     '),write(b).
a     b
 
Yes

Another way of printing spaces is by using the predicate tab/1. tab takes a number as argument and then prints as many spaces as specified by that number.

?- write(a),tab(5),write(b).
a     b
 
Yes

Another predicate useful for formatting is nl. nl tells Prolog to make a linebreak and to go on printing on the next line.

?- write(a),nl,write(b).
a
b
Yes

Here is an exercise, where you can apply what you just learned.

In the last lecture, we saw how extra arguments in DCGs can be used to build a parse tree. For example, to the query s(T,[a,man,shoots,a,woman],[]) Prolog would answer s(np(det(a),n(man)),vp(v(shoots),np(det(a),n(woman)))). This is a representation of the parse tree. It is not a very readable representation, though. Wouldn't it be nicer if Prolog printed something like

s(
  np(
     det(a)
     n(man))
  vp(
    v(shoots)
    np(
      det(a)
      n(woman))))

for example?

Write a predicate pptree/1 that takes a complex term representing a tree, such as s(np(det(a),n(man)),vp(v(shoots),np(det(a),n(woman)))), as its argument and prints a nice and readable output for this tree.

Finally, here is an exercise to practice writing operator definitions.

In the practical session of Chapter 7, you were asked to write a DCG generating propositional logic formulas. The input you had to use was a bit awkward though. The formula had to be represented as [not, '(', p, implies, q, ')']. Now, that you know about operators, you can do something a lot nicer. Write the operator definitions for the operators not, and, or, implies, so that Prolog accepts (and correctly brackets) propositional logic formulas. For example:

?- display(not(p implies q)).
not(implies(p,q)).
 
Yes
?- display(not p implies q).
implies(not(p),q)
 
Yes