assert.md

Assert and Asertions

Assertions are methods that we can use to verify our code. The idea is very simple: We pass an expression that we believe is true to a method called "assert" which then double checks the expression. If the expression is false, #assert stops our program from continuing by throwing an error.

So, the #assert method basically "asserts" that the thing we pass it is true. That's it!

Building assert()

Using the following as a basis, build an assert() method in Ruby:

• assert() takes a test and an optional message
• return true if the test is true
• error with the user's message (or a default) if the test is not true

def assert(test, msg = nil)
  ...
end

Questions:

• How does assert behave when not passed a boolean expression?
• For that matter, how what qualifies as "true" in Ruby?

Other Assertions

Using your assert() method, build assert_equal (easy!):

• assert_equal takes two expressions and an optional message
• assert_equal returns true iff the two expressions are equal
• assert_equal throws an error with the optional message if the expressions are not equal

All The Things

• write assert_nil
• write assert_match (takes a regex and a string)
• write assert_includes
• make up your own assertion

Why Do We Want This?

We will use asserts to reduce errors by verifying assumptions in our code, and as the basis for automated tests.

More Info

• Invariants
• TDD
• Deep Dive Example of TDD