# Fun with List comprehensions

I had the pleasure to present at the North Bay .NET user group this week on F#, where people asked me all sorts of great questions. At some point, we got into List comprehensions – a convenient syntax to generate lists - via an example along these lines (which returns a list of the multiple of 2, from 2 to 40):

let list =
[ for i in 1 .. 20 do yield 2 * i ]


While slightly more complex, the actual example is in essence equivalent. A question followed: how complex can the code within the brackets be?

Well, pretty much as complex as you want it to be. Take this for instance:

let list =
[
for i in 1 .. 10 do yield 2 * i
for i in 1 .. 10 do yield 3 * i
]


This will return a list of the multiples of 2 from 2 to 20, followed in the same list by the multiples of 3 from 3 to 30. Nice.

But you can go much wilder, and start putting code in there, too. For instance, we can expand the previous example a bit and morph it into a nice and concise FizzBuzz:

type Fizzbuzz = Fizz | Buzz | FizzBuzz | Number of int

let fizzBuzz n =
[
let fizzBuzzConvert number =
if number % 2 = 0 && number % 5 = 0 then FizzBuzz
elif number % 5 = 0 then Buzz
elif number % 2 = 0 then Fizz
else Number(number)

for i in 1 .. n do yield fizzBuzzConvert i
]


We declare a discriminated union, covering all the possible outcomes of FizzBuzz, declare inside the comprehension itself a function that maps an integer to a FizzBuzz result, and generate the list of results from i to n. Running this in the interactive window results in the following:

let fizzBuzz n =
[
let fizzBuzzConvert number =
if number % 2 = 0 && number % 5 = 0 then FizzBuzz
elif number % 5 = 0 then Buzz
elif number % 2 = 0 then Fizz
else Number(number)

for i in 1 .. n do yield fizzBuzzConvert i
];;

type Fizzbuzz =
| Fizz
| Buzz
| FizzBuzz
| Number of int
val fizzBuzz : int -> Fizzbuzz list

> let f = fizzBuzz 20;;

val f : Fizzbuzz list =
[Number 1; Fizz; Number 3; Fizz; Buzz; Fizz; Number 7; Fizz; Number 9;
FizzBuzz; Number 11; Fizz; Number 13; Fizz; Buzz; Fizz; Number 17; Fizz;
Number 19; FizzBuzz]


I would classify List comprehension under the “nice to have” features – it’s perfectly possible to write excellent code without it. At the same time, it’s a very, very convenient way to work with Lists, which I now miss in C#…