Attempt computation expression

Attempt.fs

open System
 
[<AutoOpen>]
module Attempt = 
    type Result<'TSuccess> = 
        | Success of 'TSuccess
        | Failure of Exception
 
    // make sure f is pure, otherwise monad laws will fail
    let (>>=) x f =
        match x with 
        | Success s  -> f s
        | Failure ex -> Failure ex
 
    let pureReturn x = Success x 
 
    let safe f = fun x -> try Success (f x) with ex -> Failure ex
 
    type AttemptBuilder() = 
        member this.Bind (x, f) = x >>= f
        member this.Return (x) = pureReturn x
    
    let attempt = new AttemptBuilder ()

Example1.fs

let step01 () : Result<int> = Success (1)
let step02 () : Result<int> = Failure (new IndexOutOfRangeException("blaargh!!!"))
let step03 () : Result<int> = Success (3)

let result = attempt {
    let! one = step01 ()
    let! two = step02 ()
    let! three = step03 ()
    return [one; two; three] |> List.sum
}
printfn "%A" result

Example2.fs

let one ()   : int = 1
let two ()   : int = 2
//let three () : int = failwith "There is NO three!"
let four ()  : int = 4
let result2 = attempt {
    let! n1 = (lift0 one) ()
    let! n2 = (lift0 two) () 
    let! n3 = (lift0 three) () // this one will throw and the computation will end here
    let! n4 = (lift0 four) ()

    return n1 + n2 + n4 // comment out line #8 to see the computation complete until this point
}
printfn "%A" result2

Example3.fs

let scaryAdd5 x = if (x = 0) then failwith "Cannot add zero!" else x + 5
let scaryAdd5M = lift1 scaryAdd5

let result3 = attempt {
    let! ten = scaryAdd5M 5
    let! fifteen = scaryAdd5M ten
    return fifteen
}

let result4 = attempt {
    let! fifteen = result3 // unpack a packed value with a let! assignment...
    let! twenty = scaryAdd5M fifteen //...and now you can use it!
    return twenty
}
printfn "%A" [result3; result4]

Forget about that lift definition, it's unnecessary. Bind seems to missing the try/catch. Your lift1 is actually map, which you can define generically in terms of bind+return thus keeping things simpler.

@maush ok I'll push the try-catch into Bind and refactor. Thanks for your suggestion :)