This explanation is based on a commented Ruby script from a friend of mine. If you want to improve the script, feel free to update it at the link.
First, note that when Ruby calls out to a shell, it typically calls /bin/sh
, not Bash. Some Bash syntax is not supported by /bin/sh
on all systems.
Here are ways to execute a shell script:
cmd = "echo 'hi'" # Sample string that can be used
Kernel#`
, commonly called backticks – `cmd`
This is like many other languages, including Bash, PHP, and Perl.
Returns the result (i.e. standard output) of the shell command.
Docs: http://ruby-doc.org/core/Kernel.html#method-i-60
value = `echo 'hi'`
value = `#{cmd}`
Built-in syntax, %x( cmd )
Following the x
character is a delimiter, which can be any character.
If the delimiter is one of the characters (
, [
, {
, or <
,
the literal consists of the characters up to the matching closing delimiter,
taking account of nested delimiter pairs. For all other delimiters, the
literal comprises the characters up to the next occurrence of the
delimiter character. String interpolation #{ ... }
is allowed.
Returns the result (i.e. standard output) of the shell command, just like the backticks.
Docs: https://docs.ruby-lang.org/en/master/syntax/literals_rdoc.html#label-Percent+Strings
value = %x( echo 'hi' )
value = %x[ #{cmd} ]
Kernel#system
Executes the given command in a subshell.
Returns true
if the command was found and run successfully, false
otherwise.
Docs: http://ruby-doc.org/core/Kernel.html#method-i-system
wasGood = system( "echo 'hi'" )
wasGood = system( cmd )
Kernel#exec
Replaces the current process by running the given external command.
Returns none, the current process is replaced and never continues.
Docs: http://ruby-doc.org/core/Kernel.html#method-i-exec
exec( "echo 'hi'" )
exec( cmd ) # Note: this will never be reached because of the line above
Here's some extra advice:
$?
, which is the same as $CHILD_STATUS
, accesses the status of the last system executed command if you use the backticks, system()
or %x{}
.
You can then access the exitstatus
and pid
properties:
$?.exitstatus
For more reading see:
Consider a simple function that adds the first N natural numbers. (e.g. sum(5) = 0 + 1 + 2 + 3 + 4 + 5 = 15
).
Here is a simple JavaScript implementation that uses recursion:
function recsum(x) {
if (x === 0) {
return 0;
} else {
return x + recsum(x - 1);
}
}
If you called recsum(5)
, this is what the JavaScript interpreter would evaluate:
recsum(5)
5 + recsum(4)
5 + (4 + recsum(3))
5 + (4 + (3 + recsum(2)))
5 + (4 + (3 + (2 + recsum(1))))
5 + (4 + (3 + (2 + (1 + recsum(0)))))
5 + (4 + (3 + (2 + (1 + 0))))
5 + (4 + (3 + (2 + 1)))
5 + (4 + (3 + 3))
5 + (4 + 6)
5 + 10
15
Note how every recursive call has to complete before the JavaScript interpreter begins to actually do the work of calculating the sum.
Here's a tail-recursive version of the same function:
function tailrecsum(x, running_total = 0) {
if (x === 0) {
return running_total;
} else {
return tailrecsum(x - 1, running_total + x);
}
}
Here's the sequence of events that would occur if you called tailrecsum(5)
, (which would effectively be tailrecsum(5, 0)
, because of the default second argument).
tailrecsum(5, 0)
tailrecsum(4, 5)
tailrecsum(3, 9)
tailrecsum(2, 12)
tailrecsum(1, 14)
tailrecsum(0, 15)
15
In the tail-recursive case, with each evaluation of the recursive call, the running_total
is updated.
Note: The original answer used examples from Python. These have been changed to JavaScript, since Python interpreters don't support tail call optimization. However, while tail call optimization is part of the ECMAScript 2015 spec, most JavaScript interpreters don't support it.
Best Answer
No, Ruby doesn't perform TCO. However, it also doesn't not perform TCO.
The Ruby Language Specification doesn't say anything about TCO. It doesn't say you have to do it, but it also doesn't say you can't do it. You just can't rely on it.
This is unlike Scheme, where the Language Specification requires that all Implementations must perform TCO. But it is also unlike Python, where Guido van Rossum has made it very clear on multiple occasions (the last time just a couple of days ago) that Python Implementations should not perform TCO.
Yukihiro Matsumoto is sympathetic to TCO, he just doesn't want to force all Implementations to support it. Unfortunately, this means that you cannot rely on TCO, or if you do, your code will no longer be portable to other Ruby Implementations.
So, some Ruby Implementations perform TCO, but most don't. YARV, for example, supports TCO, although (for the moment) you have to explicitly uncomment a line in the source code and recompile the VM, to activate TCO – in future versions it is going to be on by default, after the implementation proves stable. The Parrot Virtual Machine supports TCO natively, therefore Cardinal could quite easily support it, too. The CLR has some support for TCO, which means that IronRuby and Ruby.NET could probably do it. Rubinius could probably do it, too.
But JRuby and XRuby don't support TCO, and they probably won't, unless the JVM itself gains support for TCO. The problem is this: if you want to have a fast implementation, and fast and seamless integration with Java, then you should be stack-compatible with Java and use the JVM's stack as much as possible. You can quite easily implement TCO with trampolines or explicit continuation-passing style, but then you are no longer using the JVM stack, which means that everytime you want to call into Java or call from Java into Ruby, you have to perform some kind of conversion, which is slow. So, XRuby and JRuby chose to go with speed and Java integration over TCO and continuations (which basically have the same problem).
This applies to all implementations of Ruby that want to tightly integrate with some host platform that doesn't support TCO natively. For example, I guess MacRuby is going to have the same problem.