Part 18 - Duck Typing

Duck typing is a humorous way of describing the type non-checking system. Initially coined by Dave Thomas in the Ruby community, its premise is that (referring to a value) "if it walks like a duck, and talks like a duck, then it is a duck".

Even though Boo is a statically typed language, Duck Typing is a way to fake being a dynamic language. Duck typing allows variables to be recognized at runtime, instead of compile time. Though this can add a sense of simplicity, it does remove a large security barrier.

d as duck
d = 5 // currently set to an integer.
print d
d += 10 // It can do everything an integer does.
print d
d = "Hi there" // sets it to a string.
print d
d = d.ToUpper() // It can do everything a string does.
print d
Hi there

Duck typing is very handy if you are loading from a factory or an unpredictable dynamic library.

Do not enable duck typing by default. It should only be used in a few situations.

On a side note, The booish interpreter has duck typing enabled by default. This can be disabled by typing in interpreter.Ducky = false

Here is a practical example of where duck typing is useful.

import System.Threading

def CreateInstance(progid):
    type = System.Type.GetTypeFromProgID(progid)
    return type()

ie as duck = CreateInstance("InternetExplorer.Application")
ie.Visible = true

while ie.Busy:

document = ie.Document
print("$(document.title) is $(document.fileSize) bytes long.")


  1. Come up with another good example where duck typing is effective.

Go on to Part 19 - Using the Boo Compiler