Referential Transparency
When functions are pure, i.e. have no side effects, any time the function is mentioned in a program, we can replace it with the function body and the program behavior will not change.
1 Small example
Consider a function f with side-effects. Maybe f returns a number, but also prints something to the console. Now, consider an a variable assignment a = f(x). If a is never used, then this is dead code that should be removed. But because f is side-effects, we may end up altering the behavior of the program.
In contrast, if f had no side effects, we could perform removal automatically.
2 Related concepts
- Y Combinator
- Combinators - we can replace any name for a combinator with the combinator itself.