An executing program can run into unexpected problems. A file that it it wants to read might not exist; the disk might be full when it wants to save some data; the user may provide it with some unsuitable kind of input.
ERR: (eval):1:in `open': No such file or directory - some_file
A robust program will handle these situations sensibly and gracefully. Meeting that expectation can be an exasperating task. C programmers are expected to check the result of every system call that could possibly fail, and immediately decide what is to be done:
if (file == NULL) {
fprintf( stderr, "File doesn't exist.\n" );
exit(1);
}
bytes_read = fread( buf, 1, bytes_desired, file );
if (bytes_read != bytes_desired ) {
/* do more error handling here ... */
}
...
This is such a tiresome practice that programmers can tend to grow careless and neglect it, and the result is a program that doesn't handle exceptions well. On the other hand, doing the job right can make programs hard to read, because there is so much error handling cluttering up the meaningful code.
In ruby, as in many modern languages, we can handle exceptions for
blocks of code in a compartmentalized way, thus dealing with surprises
effectively but not unduly burdening either the programmer or anyone
else trying to read the code later. The block of code marked
with begin executes until there is an exception, which causes
control to be transferred to a block of error handling code, which is
marked with rescue. If no exception occurs, the
rescue code is not used. The following method returns
the first line of a text file, or nil if there is an
exception:
begin
file = open("some_file")
info = file.gets
file.close
info # Last thing evaluated is the return value
rescue
nil # Can't read the file? then don't return a string
end
end
There will be times when we would like to be able to creatively work around a problem. Here, if the file we want is unavailable, we try to use standard input instead:
file = open("some_file")
rescue
file = STDIN
end
begin
# ... process the input ...
rescue
# ... and deal with any other exceptions here.
end
retry can be used in the rescue code to start the
begin code over again. It lets us rewrite the previous
example a little more compactly:
begin
file = open(fname)
# ... process the input ...
rescue
fname = "STDIN"
retry
end
However, there is a flaw here. A nonexistent file will make
this code retry in an infinite loop. You need to watch out for
such pitfalls when using retry for exception processing.
Every ruby library raises an exception if any error occurs, and you
can raise exceptions explicitly in your code too. To raise an
exception, use raise. It takes one argument, which
should be a string that describes the exception. The argument is
optional but should not be omitted. It can be accessed later via
the special global variable $!.
test error
ruby> begin
| raise "test2"
| rescue
| puts "An error occurred: #{$!}"
| end
An error occurred: test2
nil