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F# State Monad implementation for n-ary tree (based on "The Zen of Stateless State" video by Brian Beckman)
open System
type Tree<'a> =
| Leaf of 'a
| Branch of Tree<'a> list
type StateMonad<'state, 'content> = StateMonad of ('state -> ('state * 'content))
let testTree =
Branch [
Leaf "Mike"
Branch [
Leaf "Car"
Leaf "Door"
Branch [
Branch [
Leaf "Woof"
Branch [
Leaf "Foo"
Leaf "Baar"
Leaf "Ding"
Leaf "ALing"
Leaf "Doh"
Branch [
Leaf "Flanders"
Leaf "Stupid"
let wrap content = StateMonad (fun state -> (state, content))
let bind monad monadMaker =
StateMonad (fun state ->
let (StateMonad monadFunction) = monad
let (state1, content1) = monadFunction state
let (StateMonad newMonad) = monadMaker content1
newMonad state1
let updateState = StateMonad (fun state -> (state + 1, state))
let rec makeMonad tree =
match tree with
| Leaf content -> bind updateState (fun currentState -> wrap <| Leaf(currentState, content))
| Branch leaves ->
let folder monadList leaf =
bind monadList (fun list -> bind (makeMonad leaf) (fun lLeaf -> wrap <| lLeaf::list))
let monadList = leaves.Tail |> List.fold folder (bind (makeMonad leaves.Head) (fun head -> wrap [ head ]))
bind monadList (fun labelledLeaves -> Branch (List.rev labelledLeaves) |> wrap)
let printTree tree =
let rec printTreeWithIndent indent tree =
let indentStr = new System.String(' ', indent)
match tree with
| Leaf content -> printfn "%s|───%A" indentStr content
| Branch leaves ->
printfn "%s|───\\" indentStr
leaves |> List.iter (printTreeWithIndent (indent + 4))
printTreeWithIndent 0 tree
let main argv =
printTree testTree
let (StateMonad (monad)) = makeMonad testTree
let state, tree = monad 0
printTree tree
Console.ReadLine() |> ignore
0 // return an integer exit code

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@halfAbee halfAbee commented Apr 16, 2015

As an early adopter of Channel 9 videos, I have always enjoyed the contributions of Brian (and Erik Meijer) as they cropped up over the years.

I thank iskeld for this splendid translation of Brian's thoughts into F#. I watched the original video when it came out, and I couldn't be bothered actually trying to run (as opposed to just viewing) the Haskell or C# code.

I have just spent most of the last year trying to get better at F# and the functional mindset, and your code above got me out of a corner on a recent stumble.

Someone at Microsoft should find a way to get Brian and Erik to promote F#. (despite E's stated views since leaving MS), I believe they both know that F# is and could be an even better force for good.

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