Created
April 24, 2014 21:54
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Монада Maybe и мотивирующий пример с вводом-выводом
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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 | |
} |
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