Functional programming - standard symbols, diagrams, etc

Question

I have a problem, which I believe to be best solved through a functional style of programming.

Coming from a very imperative background, I am used to program design involving class diagrams/descriptions, communication diagrams, state diagrams etc. These diagrams however, all imply, or are used to describe, the state of a system and the various side effects that actions have on the system.

Are there any standardised set of diagrams or mathematical symbols used in the design of functional programs, or are such programs best designed in short functional-pseudo code (given that functions will be much shorter than imperative counterparts).

Thanks, Mike

Answer 1

There's a secret trick to functional programming.

1. It's largely stateless, so the traditional imperative diagrams don't matter.

2. Most of ordinary, garden-variety math notation is also stateless.

Functional design is more like algebra than anything else. You're going to define functions, and show that the composition of those functions produces the desired result.

Diagrams aren't as necessary because functional programming is somewhat simpler than procedural programming. It's more like conventional mathematical notation. Use mathematical techniques to show that your various functions do the right things.

Answer 2

Functional programmers are more into writing equations then writing diagrams. The game is called equational reasoning and it mostly involves

• Substituting equals for equals

• Applying algebraic laws

• The occasional proof by induction

The idea is that you write really simple code that is "manifestly correct", then you use equational reasoning to turn it into something that is cleaner and/or will perform better. The master of this art is an Oxford professor named Richard Bird.

For example, if I want to simplify the Scheme expression

(append (list x) l)

I will subsitute equals for equals like crazy. Using the definition of list I get

(append (cons x '()) l)

Subsituting the body of append I have

(if (null? (cons x '())) 
    l
    (cons (car (cons x '())) (append (cdr (cons x '())) l)))

Now I have these algebraic laws:

(null? (cons a b)) == #f
(car   (cons a b)) == a
(cdr   (cons a b)) == b

and substituting equals for equals I get

(if #f
    l
    (cons x (append '() l))

With another law, (if #f e1 e2) == e2, I get

(cons x (append '() l))

And if I expend the definition of append again I get

(cons x l)

which I have proved is equal to

(append (list x) l)

Answer 3

• There is this very good article explaining Lambda Calculus using animations: To Dissect a Mockingbird: A Graphical Notation for the Lambda Calculus with Animated Reduction
• This one is very similar to the previous, but has a actual implementation: Lambda Animator

Answer 4

I don't know much about functional programming, but here are two things I have run into

• λ (lambda) is often used to denote a function
• f ο g is used to indicate function composition