Refactoring: Improving the Design of Existing Code (Addison-Wesley Signature Series (Fowler))

by Martin Fowler

It’s easier to reason about a function that will always give the same result when called with same parameter values—this is called referential transparency. If a function accesses some element in its scope that isn’t referentially transparent, then the containing function also lacks referential transparency. I can fix that by moving that element to a parameter. Although such a move will shift responsibility to the caller, there is often a lot to be gained by creating clear modules with referential transparency. A common pattern is to have modules consisting of pure functions which are wrapped by logic that handles the I/O and other variable elements of a program. I can use Replace Query with Parameter to purify parts of a program, making those parts easier to test and reason about.