3.3

Performing operations from boolean algebra is also quite straightforward in Elm.

> True || False
True

> False || False
False

> True && False
False

> True && True
True

> not True
False

> xor True False
True

> xor True True
False

> xor False False
False

• || - Boolean or - returns true if at least one input is True.
• && - Boolean and - returns true only if both options are True.
• not - Boolean negation - returns the opposite value of the input.
• xor - Boolean exclusive-or - returns True if exactly one input is True.

Some programming languages treat the number zero or null as synonymous with False. Elm doesn’t allow that, nor does it consider a non-zero value to be True. For a condition to be true, it must evaluate exactly to the value True.

> True && 0

----------------- TYPE MISMATCH -----------------------
I am struggling with this boolean operation:

4|   True && 0
             ^
Both sides of (&&) must be Bool values, but the right side is:

    number

Hint: Only Int and Float values work as numbers.

Look how helpful the error messages are. It’s one of many reasons why Elm is such a delightful language.