Types in Ruby
Conversations at work
A couple of months ago, I wrote a greedy bin packing algorithm for work. The other day, Chris decided he a needed a simpler packing algorithm, and asked me to explain how my solution worked.
“What type is files?” Chris asked as we stepped through the code.
files.each do |file|
name = file[0]
size = file[1]
# Bin packing algorithm...
end
“It’s most likely a hash or a two-dimensional array,” I said. “Probably a hash; though, I’m not really sure.”
Chris gave me a look that said, “How can you look at code you wrote and not even know what types of variables you used?”
Well, Ruby doesn’t really have types.
Duck typing
A Ruby variable holds a reference to an object of a certain type if it behaves like that type. This is commonly called duck typing, i.e. “If it walks like a duck, and it quacks like a duck, it must be a duck.”
When people are first learning Ruby, they tend to think that type and class are
intertwined. If files.class returns Hash, then files must be a hash. But
consider this:
class Files
def class
Hash
end
end
files = Files.new
if files.class == Hash
files['ruby.txt'] = 3
end
Even though files.class returns Hash, files doesn’t have a reference to a
Hash, it has a reference to a Files object. Okay, so what about using the
instance_of? method?
class Files
def instance_of?(type)
type == Hash
end
end
files = Files.new
if files.instance_of?(Hash)
files['ruby.txt'] = 3
end
That doesn’t work. instance_of? suffers from the same problem as class. When
the code is executed, it throws a error saying the []= method is missing. So
what about asking the…
http://alek.xspaces.org/2005/02/27/ruby-type-explosion