Getting the MVVM Kicked Out of Your F#'n Monads

Getting the MVVM kicked out of your F#'n Monads Richard Minerich F# MVP of the Year Senior Researcher, Bayard Rock @Rickasaurus / [email_address] Fill this space with whatever you want (graphic, logo, whatever)

Time Warp back to 6 th Grade
1 + (2 + 3) = 6 and (1 + 2) + 3 = 6
and
1 * (2 * 3) = (1 * 2) * 3

What about functions?
let addOne x = x + 1
let addTwo x = x + 2
let addThree x = x + 3
All are int -> int
let addSix x = addOne(addTwo(addThree(x)))

>> is like add for functions
let addOne x = x + 1
let addTwo x = x + 2
let addThree x = x + 3
All are int -> int
let addSix = addOne >> addTwo >> addThree

5 + 0 = 5 and 3 * 1 = 3
What would be the same thing for a function?

5 + 0 = 5 and 3 * 1 = 3
What would be the same thing for a function?
let id f = f
So,
addFive >> id = addFive

functions are like numbers
Just like (2 + 3) + 4 = 2 + (3 + 4)
let addSix = (addOne >> addTwo) >> addThree
is the same as
let addSix = addOne >> (addTwo >> addThree)
Just like 6 = 0 + 6
addSix = id >> addSix

(2 / 3) / 4 isn’t 2 / (3 / 4) but,
let divByTwo x = x / 2
let divByThree x = x / 3
let divByFour x = x / 4
let divByTwentyFour = (divByTwo >> divByThree) >> divByFour
is the same as
let divByTwentyFour = divByTwo >> (divByThree >> divByFour)
Why?

(1 - 2) - 3 isn’t 1 - (2 - 3) but,
let minusOne x = x - 1
let minusTwo x = x - 2
let minusThree x = x - 3
let minusSix = (minusOne >> minusTwo) >> minusThree
is the same as
let minusSix = minusOne >> (minusTwo >> minusThree)
Why?

(1 - 2) - 3 isn’t 1 - (2 - 3) cont.
let minusSix = minusTwo >> minusThree >> minusOne
Works great. But consider..
let minusSix =
timesTwo >> divTwo >> minusThree >> minusThree
let ohNo =
minusThree >> divTwo >> minusThree >> timesTwo

School’s out!
First class functions are in!

One Style of Abstraction
let ten = [1; 2; 3; 4] |> List.fold (fun x y -> x + y) 0
let ten = [1; 2; 3; 4] |> List.reduce (fun x y -> x + y)
let ten = [1; 2; 3; 4] |> List.reduce (+)
let ten = [1; 2; 3; 4] |> List.sum
Fold – You provide the identity
Reduce – No identity needed
Reduce – Actually, just the operator is good
Sum – Complete package, you provide nothing

It works with functions too!
let ten =
[addOne; addTwo; addThree; addFour]
|> List.fold (fun x f -> f x) 0
let ten =
[addOne; addTwo; addThree; addFour]
|> List.reduce (>>)
<| 0
But how do we abstract ideas like these over the inside of functions?

Computation Expressions
Will Blow Your Mind

Computation Expressions!
-> Each line is a lambda.
-> In normal class methods you dispatch on type, with computation expressions you dispatch on language constructs.

What can you manipulate?
Binding, Yielding and Returning
For and While
Use, Try-With and Try-Finally
Per-function
The Entire Computation
The computation can be manipulated at all of these levels.

Let’s take a look at a simple CE
type MultBuilder () =
member t.Combine (x,y) = x * y
member t.Yield (x) = x
member x.Delay(f) = f()
let mult = new MultBuilder()
mult {yield 2; yield 3}

It goes much deeper.
Return – Stuff something in a container
Bind – Pull it out
(or decide what to do it’s empty)
Computation expressions allow us to do this behind the scenes in an elegant way.

Option
type Option<'a> = | Some of 'a | None
Most easily used as:
match x with
| Some x -> x
| None -> raise “Oh god, what now!”
But we can do better.

Maybe
Maybe allows us to short circuit when something goes wrong, instead of matching again and again.

“ F# 2.0 is a book that assumes you are a professional, and as such, does not waste your time with a lot of fluff.”

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Getting the MVVM Kicked Out of Your F#'n Monads

by Richard Minerich on Mar 25, 2011

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