Created
April 24, 2014 22:03
-
-
Save shwars/11271066 to your computer and use it in GitHub Desktop.
Continuation Monad
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
// Simple computations | |
let read() = System.Int32.Parse(System.Console.ReadLine()) | |
let double x = x*2 | |
let print x = printfn "%d" x | |
(read >> double >> print)() | |
// Same computations using continuations | |
let read cont = | |
let x = System.Int32.Parse(System.Console.ReadLine()) | |
cont x | |
let double x cont = cont(x*2) | |
let print x cont = cont(printfn "%d" x) | |
read (fun x-> double x (fun x' -> print x' (fun x''->x''))) | |
// Introducing bind | |
let (>>=) a f = a f | |
(read >>= double >>= print)(fun x->x) | |
// Using F# Computational Expression Syntax | |
type ContinuationBuilder() = | |
member this.Bind (m, f) = fun c -> m (fun a -> f a c) | |
member this.Return x = fun k -> k x | |
let cont = ContinuationBuilder() | |
let res = (cont { | |
let! x = read | |
let! x' = double x | |
do! (print x') | |
}) (fun x->x) | |
// Implementins QSort using continuations | |
let qsort list = | |
let rec loop list cont = | |
match list with | |
| [] -> cont [] | |
| x::[] -> cont [x] | |
| x::xs -> | |
let l,r = | |
List.partition ((>) x) xs | |
loop l (fun ls -> | |
loop r (fun rs -> | |
cont (ls @ (x::rs)))) | |
loop list (fun x -> x) | |
qsort [6;3;89;34;67;1;38] | |
// The same, using monadic syntax | |
let qsort list = | |
let rec loop list = | |
cont { | |
match list with | |
| [] -> return [] | |
| x::xs -> | |
let l, r = List.partition ((>) x) xs | |
let! ls = loop l | |
let! rs = loop r | |
return (ls @ (x::rs)) | |
} | |
loop list (fun x -> x) | |
qsort [6;3;89;34;67;1;38] | |
// Factorial using monadic syntax | |
let fac n = | |
let rec loop n = | |
cont { | |
match n with | |
| n when n = 0I -> return 1I | |
| _ -> let! res = loop (n - 1I) | |
return n * res | |
} | |
loop n (fun x -> x) | |
fac 100I |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment