F# monads

F# monads

Monads implemented:

Maybe
Reader
Writer

Maybe monad.fs

type Maybe<'a> =
| Just of 'a
| Nothing
 
let Return x = Just x
 
let Bind mx f =
match mx with
| Just x -> f x
| Nothing -> Nothing
 
let (>>=) = Bind

Reader monad.fs

type Reader<'v> = Reader of 'v

let Return v = Reader v

let runReader mx d = 
    match mx with
    | Reader v -> v d

let Bind mx f =
    match mx with
    | Reader v -> Reader (fun z -> (v z) |> f |> runReader <| z)

let (>>=) = Bind

/////////////////////////////////////////////////////////////////////////////

let greeter =
    Return (fun n -> "hello, " + n + "!") 

let calc (s:string) =
    Return (fun (n:string) -> s.Length - n.Length)

let config = "leon"

let r = runReader (greeter >>= calc) config

Writer monad.fs

type Writer<'d, 'v> = Writer of 'd * 'v
    
let Tell d f = f d

let Return v = fun d -> Writer(d,v)

let Join d mx =
    match mx with
    | Writer(x:string,v) -> Writer(d+x, v)

let Bind mx f = 
    match mx with
    | Writer(d,v) -> f v |> Join d

let runWriter mx =
    match mx with
    | Writer(d,v) -> d,v

let (>>=) = Bind

/////////////////////////////////////////////////////////////////////////////

let half x = 
    Tell ( sprintf "I just halved %d ! " x ) <| Return ( x / 2 )

let log, result = half 8 >>= half |> runWriter

printfn "Log: %s" log
printfn "Result: %d" result