From: "Jörg W Mittag" <JoergWMittag+Ruby@...>
Date: 2010-09-15T00:10:17+09:00
Subject: Re: method! and method?

Gene Angelo wrote:
> I am trying to find the meaning of ! and ? appended to method names but
> I cannot find what they mean. Can someone explain?

They aren't appended to the method name. They are part of the method
name. A method ending in '!' or '?' means pretty much the same as a
method name ending in 'a' or 'z': it means whatever the author of that
method wants it to mean.

There is a convention, however: a method name ending in '?' usually
indicates a method that returns either a truthy or a falsy value. For
example, take the `Integer#even?` method. An integer is either even or
it's not. Or `Array#empty?`, `Numeric#zero?`, `Time#thursday?`: an
array is either empty or not, a number is either zero or not, a
specific date is either a thursday or not.

For a method name ending in '!', the convention is the following: if
there are two methods that both perform the same operation, but in a
slightly different way, then they both get the same name, but the one
which is more "surprising" gets an exclamation point appended. For
eample: `String#downcase` downcases as string. It returns a new
downcased string, but leaves the original untouched.
`String#downcase!` *also* downcases a string, but *it* destroys the
original string and *replaces* it with a downcased version.

jwm