Graphviz
Graphviz is a collection of programs for visualising graphs. Here, we conveniently use it for visualising Prolog terms and SLD trees. It is probably easier to try this out first with SWISH. In order to do that run SWISH server and point your browser to http://localhost:3050/example/graphviz_intro.swinb.
Download the file graphviz.pl. This file defines the two main predicates term/1 and sld/1. The query ?-term(Term), where Term is bound to a Prolog term, will produce a file term.dot, which defines a tree in Graphviz format. For example, the query ?-term([a,b,b,a]) produces the following Graphviz input:
digraph {
node [shape=plaintext, fontname=Courier, fontsize=12]
0 [label="."];
1 [label="a"];
0 -> 1;
2 [label="."];
0 -> 2;
3 [label="b"];
2 -> 3;
4 [label="."];
2 -> 4;
5 [label="b"];
4 -> 5;
6 [label="."];
4 -> 6;
7 [label="a"];
6 -> 7;
8 [label="[]"];
6 -> 8;
}
The command dot -T jpg -o term.jpg term.dot then produces a JPEG file with the following tree:
See here for more information about how to use Graphviz programs from the command line.
Similarly, the query ?-sld(Goal), where Goal is bound to a Prolog goal, will produce a file sld.dot, which defines a tree in Graphviz format. For example, the query ?-sld(student_of(S,peter)) produces the following tree when run through dot:
Now, try the example queries ?-term2 and ?-sld2 that have been pre-defined in graphviz.pl.
The Graphviz binaries can be found in /usr/local/ml/graphviz/2.6/bin/. You can add this to your path so that you can easily use the Graphviz commands from the command line.