Монада Maybe и мотивирующий пример с вводом-выводом

gistfile1.fs

open System

let read() =
   printf ">"
   let s = Console.ReadLine()
   try
     Some((int)s)
   with
     _ -> None

let a = read()
if a<>None then
   let b = read()
   if b<>None then printfn "Result is %d" (a.Value+b.Value)
   else printfn "Bad input"
else printfn "Bad input"


let bind a f =
   if a=None then None
   else f (a.Value)

bind (read()) (fun x -> bind (read()) (fun y -> Some(x+y)))

let (>>=) a f =
   if a=None then None
   else f (a.Value)

read() >>= (fun x -> read() >>= (fun y->Some(x+y)))


// Monadic Syntax
let read() =
   printf ">"
   let s = Console.ReadLine()
   try
     Some((int)s)
   with
     _ -> None

let (>>=) a f =
   if a=None then None
   else f (a.Value)

read() >>= (fun x -> read() >>= (fun y->Some(x+y)))

type MaybeBuilder () =
   member b.Bind(a,f) = a>>=f
   member b.Return(x) = Some(x)
   member b.Zero() = None
   member b.Combine(x,y) = x>>=(fun _ -> y)

let maybe = new MaybeBuilder()

maybe {
   let! x = read()
   let! y = read()
   return x+y
 }

maybe {
   let! x = read()
   let! y = read()
   let z = 2
   printfn "x=%d, y=%d, z=%d" x y z
   return (x+y)*z
 }