Railway oriented programming

railway.fs

// Taken from http://fsharpforfunandprofit.com/posts/recipe-part2/

// the two-track type
type Result<'TSuccess,'TFailure> = 
    | Success of 'TSuccess
    | Failure of 'TFailure

// convert a single value into a two-track result
let succeed x = 
    Success x

// convert a single value into a two-track result
let fail x = 
    Failure x

// apply either a success function or failure function
let either successFunc failureFunc twoTrackInput =
    match twoTrackInput with
    | Success s -> successFunc s
    | Failure f -> failureFunc f

// convert a switch function into a two-track function
let bind f = 
    either f fail

// pipe a two-track value into a switch function 
let (>>=) x f = 
    bind f x

// compose two switches into another switch
let (>=>) s1 s2 = 
    s1 >> bind s2

// convert a one-track function into a switch
let switch f = 
    f >> succeed

// convert a one-track function into a two-track function
let map f = 
    either (f >> succeed) fail

// convert a dead-end function into a one-track function
let tee f x = 
    f x; x 

// convert a one-track function into a switch with exception handling
let tryCatch f exnHandler x =
    try
        f x |> succeed
    with
    | ex -> exnHandler ex |> fail

// convert two one-track functions into a two-track function
let doubleMap successFunc failureFunc =
    either (successFunc >> succeed) (failureFunc >> fail)

// add two switches in parallel
let plus addSuccess addFailure switch1 switch2 x = 
    match (switch1 x),(switch2 x) with
    | Success s1,Success s2 -> Success (addSuccess s1 s2)
    | Failure f1,Success _  -> Failure f1
    | Success _ ,Failure f2 -> Failure f2
    | Failure f1,Failure f2 -> Failure (addFailure f1 f2)