A pure function is a function that depends only on its declared input parameters and its algorithm to produce its output

It does not read any other values from the world outside of the function’s scope and it does not modify any values in the outside world

A Pure Function has no side effects on system resource

• Compiler can discard or optimize out the return value if it was not used in an expression
• The same result is returned for same parameters i.e. one-to-one correspondence or mapping. This can enable caching optimizations such as memoization
• Two independent functions can be evaluated out of order, or in parallel without worrying about side-effects
• Same strategy can be applied by the compiler if the entire program has many independent functions i.e. many parts of the whole program can run in parrallel, withoug baking-in any paralleism logic!

A mantra for writing pure functions : Output depends only on input

Mathematical functions (scala.math._) are great examples of pure functions because their output depends only on input

• abs
• ceil
• max
• min

Because a Scala String is immutable, every method available to a String is a pure function, including:

• charAt
• isEmpty
• length
• substring

Many methods that are available on Scala’s collections’ classes fit the definition of a pure function, including:

• drop
• filter
• map
• reduce

Conversely, the collections’ classes, the foreach method is impure. foreach is used only for its side effects: def foreach(f: (A) => Unit): Unit

Date and time related methods like getDayOfWeek, getHour, and getMinute are all impure because their output depends on something other than their inputs. Their results rely on some form of hidden I/O

Methods on the scala.util.Random class like nextInt are also impure because their output depends on something other than their inputs