Seven Languages in Seven Weeks - Prolog Day 2 Self-Study

More fun with prolog! This self-study section consists in implementing some list operations.

Reverse the elements of a list

rev([], []).
rev([Head|Tail], List) :- append(X, [Head], List), rev(Tail, X).

Find the smallest element of a list.

min([X], X).
min([Head1|[Head2|Tail]], X) :- Head1 =< Head2, min([Head1|Tail], X).
min([Head1|[Head2|Tail]], X) :- Head1 > Head2, min([Head2|Tail], X).

Sort the elements of a list.

insert(X, [], [X]).
insert(X, [H|T], [X,H|T]) :- X =< H.
insert(X, [H|T], [H|Rest]) :- X > H, insert(X, T, Rest).

isort([], Acc, Acc).
isort([H|T], Acc, Sorted) :- insert(H, Acc, Acc2), isort(T, Acc2, Sorted).

Seven Languages in Seven Weeks - Prolog Day 1 Self-Study

For this book's section about prolog, I'll be using SWI-Prolog, as I'm currently getting an error installing the gnu-prolog macport. Here's the link to the SWI-Prolog online reference.

Make a simple knowledge base. Represent some of your favorite books and authors.

% books.pl
book(sevenLanguagesInSevenWeeks).
book(cleanCode).
book(cleanCoder).
book(programmingInScala).
author(odersky).
author(tate).
author(martin).
wrote(odersky, programmingInScala).
wrote(tate, sevenLanguagesInSevenWeeks).
wrote(martin, cleanCode).
wrote(martin, cleanCoder).

Find all books in your knowledge base written by one author.

?- ['books'].
% books compiled 0.00 sec, 13 clauses
true.

?- wrote(martin, X).
X = cleanCode ;
X = cleanCoder.

Make a knowledge base representing musicians and instruments. Also represent musicians and their genre of music.

% music.pl
instrument(flea, bass).
instrument(hendrix, guitar).
instrument(hancock, piano).
instrument(cash, guitar).
genre(flea, funk).
genre(hendrix, blues).
genre(hancock, jazz).
genre(cash, country).

Find all musicians who play the guitar.

?- ['music'].
% music compiled 0.00 sec, 10 clauses
true.

?- instrument(X, guitar).
X = hendrix ;
X = cash.

How to get a List of Installed JVMs on a Mac

I got this tip from a fellow worker:

$ /usr/libexec/java_home -V

This command will get you the list of installed Java Virtual Machines installed on your Mac OS X system.

Inspecting the Process Environment Variables with GDB

While trying to solve the 4th level of the vortex wargame, I found it was necessary to learn how to inspect the location and content of the environment variables within the process memory.

GDB has built-in commands to inspect the process environment, see the GDB manual. You can either list all environment variables or a specific one (e.g. FOOBAR) using the following commands, which will output their values:

(gdb) show environment
(gdb) show environment FOOBAR

In order to locate the environment variables within the process memory, you can query the variable char** environ (see the libc reference and this entry on stack overflow):

(gdb) x/s *((char **)environ)

This will print the location of the first environment variable and its representation as string. To print the next variables, simply add an offset to the variable:

(gdb) x/s *((char **)environ + 1)

I also found these links to be useful:

• GDB macros by IBM developers resources
• Some more GDB tips & tricks