Duck Typing is a bit of a buzz word. If you don't know what it is, it's likely that you've used it, or at least read code that uses it, without knowing it.
There's a million posts on duck typing. Most try to get all meta and use an example of a class called "Duck", which does nothing to help explain the concept. This post is what I wished I could read straight up.
The first word of duck typing is a reference to the phrase "If it walks like a duck, and quacks like a duck, it's a duck". That'll make more sense as we go.
The second word of duck typing refers to the type of an object. Consider the code below:
first_name = "Fred"
# 'first_name' is an object of type String
test_scores = [97, 45, 86]
# 'test_scores' is an object of type Array
birthday = Date.new(2001,2,3)
# 'birthday' is an object of type Date
The type refers to the class that an object is an instance of.
Wikipaedia states: "In duck typing, a programmer is only concerned with ensuring that objects behave as demanded of them in a given context, rather than ensuring that they are of a specific type."
So, when we're 'duck typing', then we're not concerned with exactly what type of object we have (String, Array, Date, etc). If it behaves like the object we want for our specific purposes, then that's good enough. (If it 'talks like a duck', it's good enough.)
That will make a LOT more sense after an example.
Let's say we want to implement a method, show_length which will tell us how long a string is. Eg:
name = "Joss"
show_length(name)
# output: "It has a length of 4"
First we will NOT use duck typing, then we'll show the same example using duck typing.
def show_length(object)
if object.class == String
puts "It has a length of #{object.length}"
elsif
puts "You passed a #{object.class}. We wanted a String."
end
end
Note that we strictly check the type of the object passed in. This is NOT duck typing. So, example input / output would be as follows:
show_length("Fred")
# Outputs: It has a length of 4
show_length([1, 2, 3])
# Outputs: You passed a Array. We wanted a String.
show_length(Date.new(2001,2,3))
# Outputs: You passed a Date. We wanted a String.
But maybe it was overkill to enforce that the parameter was a String. When we passed in the array, it should have just told us the length of the array. In fact, lets not bother checking the type of the object at all; if it responds to the length method, it's good enough for us. (If it walks like a duck, and quacks like a duck, it's a duck.)
So, let's use duck typing…
def show_length(object)
if object.respond_to?(:length)
puts "It has a length of #{object.length}"
elsif
puts "You passed a #{object.class}. We wanted something with a length."
end
end
Note that we DO NOT check the type of the object passed in. So, example input / output would be as follows:
show_length("Fred")
# Outputs: It has a length of 4
show_length([1, 2, 3])
# Outputs: It has a length of 3
show_length(Date.new(2001,2,3))
# Outputs: You passed a Date. We wanted something with a length.
See how we don't check the type of the object; we just check that it's good enough for this particular context. If it responds to the length method, it'll do.
As Wikipaedia states: "In duck typing, a programmer is only concerned with ensuring that objects behave as demanded of them in a given context, rather than ensuring that they are of a specific type."
In this context, we just care that we can display the object's length. We don't care about the specific type of object.
Note that in this example of soft duck typing, we do at least check that the parameter passed in has a length method. We might decide we want to save ourself a few lines, and just assume the caller will pass the correct thing.
Let's do that now…
def show_length(object)
puts "It has a length of #{object.length}"
end
That's concise! We perform NO CHECKS on the object passed in. We just assume it'll have a length method. So, example input / output would be as follows:
show_length("Fred")
# Outputs: It has a length of 4
show_length([1, 2, 3])
# Outputs: It has a length of 3
show_length(Date.new(2001,2,3))
# Gives a NoMethodError, since Date's don't have a length
Now you know the basics, this blog post has some great examples of duck typing applied in real life scenarios.