Functional Programming

Memoization

Imagine you have a long running function that you’d like to avoid running unnecessarily. For the purposes of this post you’ll have to suspend disbelief and pretend that negating a number is an expensive task. This example prints out a message so you can see when it actually gets called.

let Negate n =
    printfn "Negating '%A' this is hard work" n
    -n
 
val Negate : int -> int

> Negate 5;;
Negating '5' this is hard work
val it : int = -5

Now, let’s use that function when writing another. This function takes two numbers, negates the first, then adds the second.

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Unfolding Sequences

In my last post I worked through an example that finds the range of numbers that sum to a target value, or gets as close as possible without exceeding the target. I mentioned that the solution felt a little too like the loopy code I would have written in non-functional languages. I felt that there might be a more “functional” way of solving the problem, but I didn’t know what it was.

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Iterating, Incrementing, and Accumulating

Another F# session this evening and some more deliberate practice of Functional Thinking. To be fair, this post isn’t really about anything new. If you’ve ever used recursion, even in non-functional languages, this will be old news. If you are new to Functional Programming and/or recursion then this may be useful.

Here’s a really simple function. It accepts a number n and sums all the numbers from 1 to n.

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Partial Application

Warning, novice functional thinker here. If you know your stuff, what follows may cause distress.

I was messing with F# last night and I got a gentle reminder that I’m still a long way from thinking functionally, it still takes a lot of effort.

I started with this

let evens r = List.filter (fun x -> x % 2 = 0) r
> evens [0..10];;

val it : int list = [0; 2; 4; 6; 8; 10]

Simple enough. Even numbers are multiples of 2. Rekindling childhood memories of typing code from books by Tim Hartnell into a ZX81, I did what I would have done then. I tried to make the code do something slightly more complicated. What about a function that can find multiples of any number, not just 2.

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Getting Functional with F# and The Game Of Life

One session at NDC that really kicked my grasp of functional programming up a few notches was Vagif Abilov’s discussion of Conway’s Game Of Life using F#.

I’m not going to rehash the rules of Game Of Life here, if you aren’t familiar with them then read this.

Vagif’s source code is on github and his slides are on slideshare. His stuff is well worth a look, but don’t look yet.

If you want to really play along with the spirit of this post, stop reading now, and go and code up Conway’s game of life in the object oriented language of your choice. I’ll be here when you get back.

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