When I started learning Clojure, I thought I knew what functional programming was but I’ve learned that the functional paradigm is now more than I expected.
Everyone agrees that it’s a computational model based on evaluation of mathematical functions, which return values. This is generally contrasted with imperative programming languages such as FORTRAN, C, JavaScript or Python, which are also procedural and some of which are object-oriented but may make functional coding possible, in a hybrid style. I wouldn’t recommend learning functional concepts in a language that gives you short-cuts to stray back to more familiar territory.
Clojure is a member of the Lisp family, first specified in 1958. The unusual feature of Lisps is their homoiconicity – code and data are the same thing. Learning Clojure has informed my thinking about business process change.
Some modern, functional languages such as Clojure use immutable data whenever possible, to eliminate side-effects. This allows better use of multi-core processors but requires a complete change in thinking, as well as programming style. ‘Variables’ are replaced by fixed ‘values’, so loops have to be replaced by recursive functions. New data can be created but it doesn’t replace old data. Yesterday’s “today’s date” isn’t automatically wiped when we decide today has happened.
Objects with their methods and local data were designed for simulating the current state of real-world objects by changing (mutating) object data state. The object model, like relational databases, has no inbuilt representation of time. Functional programming splits these objects back into separate functions and data structures and because values can’t change, they may be transformed by flowing through networks of functions, some recursive, to keep doing something until a condition is satisfied. Eventually, code must have a side effect, to tell us the answer.
Rather than computation being a conditional to-do list with data being moved between boxes, it becomes a flow of data through a network of ‘computing machines’; and the data and machines can be transformed into each other.
I hear that map, reduce & filter data transformation functions will change my world again.
On The Outside 