https://en.wikipedia.org/wiki/Referential_transparency https://en.wikipedia.org/wiki/Referential_transparency > An expression is called referentially transpare

https://en.wikipedia.org/wiki/Referential_transparency https://en.wikipedia.org/wiki/Referential_transparency > An expression is called referentially transparent if it can be replaced with its corresponding value (and vice-versa) without changing the program's behavior. This requires that the expression be pure, that is to say the expression value must be the same for the same inputs and its evaluation must have no side effects. > In mathematics all function applications are referentially transparent, by the definition of what constitutes a mathematical function. However, this is not always the case in programming, where the terms procedure and method are used to avoid misleading connotations. In functional programming only referentially transparent functions are considered. > The importance of referential transparency is that it allows the programmer and the compiler to reason about program behavior as a rewrite system. This can help in proving correctness, simplifying an algorithm, assisting in modifying code without breaking it, or optimizing code by means of memoization, common subexpression elimination, lazy evaluation, or parallelization. > The concept seems to have originated in Alfred North Whitehead and Bertrand Russell's Principia Mathematica (1910–13). It was adopted in analytical philosophy by Willard Van Orman Quine.