Referential transparency
From Freepedia
In computer programming, a referentially transparent function is one that, given the same parameter(s), always returns the same result.
While in mathematics all functions are referentially transparent, in programming this is not always the case. For example, take a function that takes no parameters and returns input from the keyboard. A call to this function might be GetInput(). The return value of GetInput() depends on what the user feels like typing in, so multiple calls to GetInput() with identical parameters (the empty list) may return different results.
A more subtle example is that of a function that uses a global variable (or a dynamically scoped variable, or a lexical closure) to help it compute its results. Since this variable is not passed as a parameter but can be altered, the results of subsequent calls to the function can differ even if the parameters are identical. (In pure functional programming, assignment is not allowed; thus a function that uses global (or dynamically scoped) variables is still referentially transparent, since these variables cannot change.)
The importance of referential transparency is that it allows a programmer to reason about program behavior. This can help in proving correctness, finding bugs that could not be found through testing, simplifying an algorithm, assisting in modifying code without breaking it, or optimizing code by means of memoization.
Some functional programming languages enforce referential transparency for all functions.
Referential transparency will cause as a side effect that no difference is made or recognised between a reference to a thing and the corresponding thing itself. Without referential transparency, such difference can be easily made and utilized in programs.
