Introduction

Pure Functions

Functions that have no side effect, meaning they return a value without,

• modifying a variable
• modifying datastructure in place
• setting a field
• throwing exception or haulting
• i/o operation

Function in Scala

Function f with input type A and output type B is represented as A => B read as 'a to b'. This function f relates every value of type A to exactly one value of type B determined soly by the value of A. Any change of state of internal or external process is irrelavent to computation of f(a) - remember definition of pure function.

Function

-block of code with no side effects.

Procedure

-parameterized block of code that may have side effects.

Referential transperancy

• An expression e is referentially transparent if, for all programs p, all occurrences of e in p can be replaced by the result of evaluating e without affecting the meaning of p. A function f is pure if the expression f(x) is referentially transparent for all referentially transparent x.

• a fancy way of saying, if you replace an expression in a program with it's value, the program remains unchanged.

E.g:

• String reverse is referrentially transparent

  val x = "Hello"

  val y = x.reverse

  val z = x.reverse

  y and z are same because reverse on input x produces exactly one output

• StringBuilder's append is not referentially transparent

  val x = new StringBuilder("Hello")

  val y = x.append(" World") // y is "Hello World" and x is also "Hello World"

  val z = x.append(" World") // z and x are both "Hellow World World". Note that x is no longer same after the call to append. The output of append on x(function invoked multiple times) does not produce exactly one output